Although these securities have been registered with the Securities and Exchange Commission, the Commission has not approved or disapproved any shares offered in this prospectus or determined whether this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Acorn^® is a trademark owned and registered by Columbia Acorn Trust, Columbia, Columbia Management^® and Columbia Management Services, Inc. are service marks owned and/or registered by Bank of America

Columbia Acorn Fund (Acorn^® Fund) seeks to provide long-term growth of capital.

Acorn Fund invests generally in the stocks of small- and medium-sized companies. The Fund generally invests in the stocks of companies with market capitalizations of less than $5 billion at the time of initial purchase. As long as a stock continues to meet the Fund's other investment criteria, the Fund may choose to hold the stock even if the issuing company's capitalization grows beyond a $5 billion capitalization.

Acorn Fund believes that these smaller companies, which are not as well known by financial analysts, may offer higher return potential than the stocks of larger companies.

• A stock price the Fund's investment adviser believes is reasonable relative to the assets and earning power of the company.

Acorn Fund invests the majority of its assets in U.S. companies, but also may invest up to 33% of its assets in companies outside the United States in developed markets (for example, Japan, Canada and the United Kingdom) and emerging markets (for example, Mexico, Brazil and Korea).

Additional strategies that are not principal investment strategies and the risks associated with them are described later in this prospectus under "Other Investment Strategies and Risks."

The principal risks of investing in the Fund are described below. There are many circumstances (including additional risks that are not described here) which could prevent the Fund from achieving its investment goal. You may lose money by investing in the Fund.

Management risk means that the adviser's investment decisions might produce losses or cause the Fund to underperform when compared to other funds with a similar investment goal. Market risk means that security prices in a market, sector or industry may fall, reducing the value of your investment. Because of management and market risk, there is no guarantee that the Fund will achieve its investment goal or perform favorably among comparable funds.

Since the Fund purchases equity securities, it is subject to equity risk. This is the risk that stock prices will fall over short or extended periods of time. Although the stock market historically has outperformed other asset classes over the long term, the stock market tends to move in cycles. Individual stock prices may fluctuate drastically from day-to-day and may underperform other asset classes over an extended period of time. Individual companies may report poor results or be negatively affected by industry and/or economic trends and developments. The prices of securities issued by such companies may suffer a decline in response. These price movements may result from factors affecting individual companies, industries or the securities market as a whole.

Because the Fund invests in stocks, the price of its shares — its net asset value (NAV) per share — fluctuates daily in response to changes in the market value of stocks.

Sector risk is inherent in the Fund's investment strategy. Companies that are in different but closely related industries are sometimes described as being in the same broad economic sector. The values of stocks of different companies in a market sector may be similarly affected by particular economic or market events. Although the Fund does not intend to focus on any particular sector, at times the Fund may have a large portion of its assets invested in a particular sector.

Foreign securities are subject to special risks. Foreign markets can be extremely volatile. Fluctuations in currency exchange rates may impact the dollar value of foreign securities denominated in foreign currencies and U.S. dollars, without a change in the intrinsic value of those securities. The liquidity of foreign securities may be more limited than that of domestic securities, which means that the Fund may, at times, be unable to sell foreign securities at desirable prices. Brokerage commissions, custodial fees and other fees are generally higher for foreign investments. In addition, foreign governments may impose withholding taxes that would reduce the amount of income and capital gains available to distribute to shareholders. Other risks include possible delays in the settlement of transactions or in the notification of income; less publicly available information about companies; the impact of political, social or diplomatic events; possible seizure, expropriation or nationalization of the company or its assets; and possible imposition of currency exchange controls.

Investments in emerging markets are subject to additional risk. The risks of foreign investments are typically increased in less developed countries, which are sometimes referred to as emerging markets. For example, political and economic structures in these countries may be new and developing rapidly, which may cause instability. These countries are also more likely to experience high levels of inflation, deflation or currency devaluations, which could hurt their economies and securities markets.

Small- or mid-cap companies may be more susceptible to market downturns, and their prices could be more volatile than large-cap companies. These companies are more likely than larger companies to have limited product lines, operating histories, markets or financial resources. They may depend heavily on a small management team and may trade less frequently, may trade in smaller volumes and may fluctuate more sharply in price than stocks of larger companies. In addition, such companies may not be widely followed by the investment community, which can lower the demand for their stocks.

An investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

A description of the Fund's policies and procedures with respect to the disclosure of its portfolio securities is available in the Fund's Statement of Additional Information (SAI), which can be obtained at www.columbiafunds.com or by calling 1-800-345-6611.

The Fund posts its complete portfolio holdings as of month-end at www.columbiafunds.com approximately 30-40 calendar days after such month-end. Additionally, the Fund discloses its top 15 month-end portfolio holdings at www.columbiafunds.com on or about 15 calendar days following such month-end.

Once posted, the above information will remain available on the website until at least the date on which the Fund files a Form N-CSR or Form N-Q for the period that includes the date as of which the information is current.

The bar chart below shows the Fund's calendar-year total returns (before taxes) for its Class A shares, excluding sales charges. The performance table following the bar chart shows how the Fund's average annual total returns for Class A, B and C shares, including sales charges, compare with broad measures of market performance for one year, five years and the life of the share class. The chart and table are intended to illustrate some of the risks of investing in the Fund by showing changes in the Fund's performance from year to year. All returns include the reinvestment of dividends and distributions. As with all mutual funds, past performance (before and after taxes) does not predict the Fund's future performance.

Calendar Year Total Returns show the Fund's Class A share performance for each completed calendar year since the Class commenced operations. Those returns include the effects of Fund expenses, but not the effects of sales charges paid directly by a shareholder. If sales charges were included, these returns would be lower.

Average Annual Total Returns are a measure of the Fund's average performance over the past one year, five years and the life of the share class. The table shows returns of each share class and includes the effects of both Fund expenses and current sales charges. Class B share returns do not reflect Class A share returns after conversion of Class B shares to Class A shares — see the section "Your Account — Sales Charges (Commissions)."

The Fund's returns are compared to the Russell 2500 Index (Russell 2500), Standard & Poor's 500 Index (S&P 500 Index) and the Russell 2000^® Index (Russell 2000^®). The Russell 2500, the Fund's primary benchmark, is a market-weighted index of the 2500 smallest companies that is formed by taking the 3000 largest U.S. companies and eliminating the largest 500 of those companies. The S&P 500 Index is a broad market-weighted average of 500 widely-held large capitalization U.S. stocks. The Russell 2000^® is a market-weighted index of the 2000 smallest companies of the 3000 largest U.S. companies based on market capitalization. All third-party trademarks are the property of their owners. Unlike the Fund, indices are not investments, do not incur fees, expenses or taxes, and are not professionally managed.

For the periods shown in bar chart:
Best quarter: 2nd quarter 2003, +19.85%
Worst quarter: 3rd quarter 2001, -18.69%

After-tax returns are calculated using the highest historical individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on each investor's own tax situation and may differ from those shown. After-tax returns may not be relevant to investors who hold Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

Average Annual Total Returns — for periods ended December 31, 2006⁽¹⁾

(1) Performance may reflect any voluntary waiver or reimbursement of Fund expenses by the investment adviser or its affiliates. Absent these waivers or reimbursement arrangements, performance results would have been lower.

(2) The inception date for Class A, Class B and Class C shares is October 16, 2000.

Expenses are one of several factors to consider before you invest in a mutual fund. The tables below describe the commissions and other expenses you may pay when you buy, hold and sell shares of the Fund.

Sales Charges (Commissions) are paid directly by shareholders to Columbia Management Distributors, Inc., the Fund's distributor, who compensates your financial advisor.

Annual Fund Operating Expenses are paid by the Fund. They include management and administration fees, 12b-1 fees and other expenses that generally include, but are not limited to, transfer agency, custody, and legal fees as well as costs related to state registration and printing of Fund documents. The specific fees and expenses that make up the Fund's other expenses will vary from time to time and may include fees or expenses not described here. The Fund will likely incur portfolio transaction costs that are in addition to the total annual fund operating expenses disclosed in the fee table. These transaction costs are made up of all costs that are associated with trading securities for the Fund's portfolio and include, but are not limited to, brokerage commissions and market spreads, as well as potential changes to the price of a security due to the Fund's efforts to purchase or sell it. While certain elements of transaction costs are readily identifiable and quantifiable, other elements that can make up a significant amount of the Fund's transaction costs are not. Higher transaction costs reduce the Fund's returns.

(1) This charge applies only to certain purchases of Class A shares bought without an initial sales charge that are sold within 12 months of purchase.

(1) The Fund pays an investment advisory fee of 0.64%. The Fund and its adviser have implemented a breakpoint schedule for the Fund's investment advisory fees. The investment advisory fee rates charged to the Fund will decline as Fund assets grow and will continue to be based on a percentage of the Fund's average daily assets. The annual fee schedule for the Fund is as follows: 0.74% for assets up to $700 million; 0.69% for assets in excess of $700 million and up to $2 billion; 0.64% for assets in excess of $2 billion and up to $6 billion; and 0.63% for assets in excess of $6 billion. In addition to the management fee, the Fund and the other funds of Columbia Acorn Trust (the "Trust") pay the adviser an administrative fee based on the average daily aggregate net assets of the Trust at the following annual rates: 0.05% of net assets up to $8 billion; 0.04% of the next $8 billion; and 0.03% of net assets in excess of $16 billion. Based on the Trust's average daily net assets as of December 31, 2006, the administrative fee would be at a rate of 0.04% and is included in "Other expenses."

(2) The distribution and service (12b-1) fees for Class B shares have been restated to reflect a reduction of the distribution fee rate by 0.10%, effective August 1, 2006.

(3) "Other expenses" have been restated to reflect contractual changes to the transfer agency fees paid by the Fund.

Example Expenses help you compare the cost of investing in the Fund to the cost of investing in other mutual funds. It uses the following hypothetical conditions:

† For example only, since the minimum initial investment in the Fund is $75,000.

Example Expenses for a $10,000 investment (your actual costs may be higher or lower)

See the Appendix for additional hypothetical investment and expense information.

The Fund offers three classes of shares in this prospectus – Class A, Class B and Class C. Each share class has its own commission and expense structure that affects the investment return of that class. Determining which share class is best for you depends on various factors, including the amount you are investing, how long you expect to hold your investment and your personal situation.

You should consult with your financial advisor before deciding which share class is most appropriate for you. The following considerations should be part of your assessment:

• You will pay a commission if you buy Class A, B or C shares, either at the time of investment or redemption, or through ongoing distribution commission payments (Rule 12b-1 fees) made from your investment over time, or both.

• Class A shares require an up-front commission payment, but pay lower ongoing Rule 12b-1 fees than Class B and Class C shares. Class B and Class C shares have no up-front commission paid by the shareholder but pay higher ongoing commissions (Rule 12b-1 fees), a deferred commission is imposed on a redemption of Class B shares within six years after purchase and a deferred commission is imposed on a redemption of Class C shares within one year of purchase. The differential between classes will vary depending on the actual investment return for any given period.

• Class A shares can be more advantageous to an investor who intends to hold the shares for several years and invests an amount large enough to qualify for a reduced rate of initial sales commission, taking into account the commissions that investor may pay when redeeming shares of other classes. To project whether Class A shares might be more advantageous to you, you should consider, among other factors, the time period you intend to hold the shares, your initial sales commission and a comparison of the total annual expenses of Class A shares to the expenses of other share classes. Class A shares pay an ongoing annual commission (Rule 12b-1 fee) of up to 0.25% of the average value of the shares.

• Class B shares are no longer offered to new investors. A Class B shareholder of the Fund whose account has a value of less than $50,000 (including for this purpose the combined value of all eligible accounts maintained by that shareholder that would be used to determine eligibility for reduced sales charges on Class A shares) may purchase additional Class B shares of the Fund to increase the account value up to a maximum of $49,999; any additional investment by the shareholder in that Fund will be invested in Class A shares of the Fund, without regard to the normal investment minimum for Class A shares, but will be subject to the applicable Class A shares up-front commission. Class B shares pay an ongoing annual commission (Rule 12b-1 fee) of up to 0.75% of the average value of the shares and automatically convert to Class A shares after eight years.

• Class C shares are generally more advantageous to an investor who intends to hold shares for only a few years. Class C shares pay an ongoing annual commission (Rule 12b-1 fee) of up to 1.00% of the average value of the shares.

• If you invest $1 million or more, you can purchase Class A shares but not Class C shares.

The Fund also offers an additional class of shares, Class Z shares, exclusively to certain institutional and other eligible investors. Class Z shares are made available through a separate prospectus provided to eligible institutional and other investors.

Your financial advisor can help you establish an appropriate investment portfolio, buy shares and monitor your investments. When the Fund receives your purchase request in "good form," your shares will be bought at the next calculated public offering price. "Good form" means that the Fund's transfer agent has all information and documentation it deems necessary to effect your order. For example "good form" may mean that you have properly placed your order with your financial advisor or the Fund's transfer agent has received your completed application, including all necessary signatures. The Fund reserves the right to refuse a purchase order for any reason, including if the Fund believes that doing so would be in the best interest of the Fund and its shareholders. The USA PATRIOT Act may require us to obtain certain personal information from you that we will use to verify your identity. If you do not provide the information, we may not be able to open your account. If we are unable to verify your customer information, we reserve the right to close your account or take such other steps as we deem reasonable.

The investment minimum for initial investments (by purchase, exchange or certain transfers) of Class A, B and C shares is $75,000. The investment minimum is applied at the class level. For group retirement plans, the investment minimum is determined based on the amount of the plan's investment rather than that of its individual participants. The Fund may establish exclusions from the investment minimum from time to time that it deems appropriate and consistent with the interests of shareholders. Please see the SAI for details on these exclusions. The Fund reserves the right to change these investment minimums.

The Fund also reserves the right to refuse a purchase order for any reason, including if it believes that doing so would be in the best interest of the Fund and its shareholders. Finally, pursuant to the Trust's Agreement and Declaration of Trust, the Fund reserves the right to redeem your shares if your account falls below the minimum investment requirements.

You may be subject to an initial sales charge when you purchase, or a contingent deferred sales charge (CDSC) when you sell, shares of the Fund. These sales charges are described below. In certain circumstances, the sales charge may be reduced or waived, as described below and in the SAI.

Rule 12b-1 Plan The Fund has adopted a plan under Rule 12b-1 under the Investment Company Act that permits it to pay its distributor ongoing sales and marketing fees (commissions) to support the sale and distribution of Class A, B and C shares and certain services provided to you by your financial advisor, and your financial advisor may receive all or a portion of those fees attributable to your shares. The annual fee, as a percentage of the value of the shares, is normally 0.25% for Class A, 0.75% for Class B and 1.00% for Class C shares. These fees are paid out of the assets of these share classes. Over time, these fees reduce the return on your investment. Class B shares automatically convert to Class A shares after eight years, eliminating a portion of the Rule 12b-1 fee thereafter.

The Fund's distributor, investment adviser or their affiliates may make payments, from their own resources, to certain financial intermediaries, including other Bank of America affiliates, for marketing support services. For purposes of this section the term "financial intermediary" includes any broker, dealer, bank, bank trust department, registered investment adviser, financial planner, retirement plan or other third-party administrator and any other institution having a selling, services or any similar agreement with the Fund's distributor, investment adviser or one of their affiliates. These payments are generally based upon one or more of the following factors: average net assets of the mutual funds distributed by the Fund's distributor attributable to that financial intermediary, gross sales of the mutual funds distributed by the Fund's distributor attributable to that financial intermediary, reimbursement of ticket charges (fees that a financial intermediary firm charges its representatives for effecting transactions in fund shares) or a negotiated lump sum payment.

While the financial arrangements may vary for each financial intermediary, the payments to any one financial intermediary are generally expected to be between 0.05% and 0.35% on an annual basis for payments based on average net assets of the Fund attributable to the financial intermediary, and between 0.10% and 0.25% on an annual basis for firms receiving a payment based on gross sales of the Fund attributable to the financial intermediary. The Fund's distributor, investment adviser or their affiliates may make payments in materially larger amounts or on a basis materially different from those described above when dealing with other affiliates of Bank of America. Such increased payments to the other Bank of America affiliate may enable the other Bank of America affiliate to offset credits that it may provide to its customers in order to avoid having such customers pay fees to multiple Bank of America entities in connection with the customer's investment in the Fund.

Payments also may be made to certain financial intermediaries, including other Bank of America affiliates, that provide investor services to retirement plans and other investment programs to compensate financial intermediaries for services they provide to such programs, including, but not limited to, sub-accounting, sub-transfer agency, similar shareholder or participant recordkeeping, shareholder or participant reporting, or shareholder or participant transaction processing. These payments for investor servicing support vary by financial intermediary but generally are not expected, with certain limited exceptions, to exceed 0.35% of the total Fund assets for Class A, B and C shares in the program on an annual basis.

The Trust's Board of Trustees has authorized the Fund to pay to any intermediary up to 0.05% of the average aggregate value of Fund shares in the intermediary's program on an annual basis for providing investor services. Charges of an intermediary in excess of that paid by the Fund will be borne by the Fund's distributor or its affiliates.

The Fund's distributor, investment adviser or their affiliates may make other payments or allow promotional incentives to financial intermediaries to the extent permitted by the rules of the Securities and Exchange Commission, the National Association of Securities Dealers and by other applicable laws and regulations.

Amounts paid by the Fund's distributor, investment adviser or their affiliates are paid out of the distributor's, investment adviser's or their affiliates' own resources and do not increase the amount paid by you or the Fund. You can find further details about the payments made by the Fund's distributor, investment adviser or their affiliates and the services provided by financial intermediaries as well as a list of the financial intermediaries to which the Fund's distributor, investment adviser or their affiliates have agreed to make marketing support payments in the Fund's SAI, which can be obtained at www.columbiafunds.com or by calling 1-800-345-6611. Your financial intermediary may charge you fees or commissions in addition to those disclosed in this prospectus. You can ask your financial intermediary for information about any payments it receives from the Fund's distributor, investment adviser and their affiliates and any services your financial intermediary provides, as well as fees and/or commissions it charges. In addition, depending on the financial arrangement in place at any particular time, a financial intermediary and its financial consultants also may have a financial incentive for recommending a particular fund or share class over others. You should consult with your financial adviser and review carefully any disclosure by the financial intermediary as to compensation received by your financial adviser.

sales charge. A portion of the sales charge is paid as a commission to your financial advisor on your purchase of Class A shares. The amount of the sales charge differs depending on the amount you invest as shown in the table below.

* Mutual fund wrap programs and group retirement plans that invest $75,000 or more in Class A shares of the Fund will not be subject to a sales charge.

** Initial purchases of less than $75,000 are no longer accepted. However, for accounts opened prior to February 3, 2006, subsequent investments that do not meet the Fund's current investment minimum will be accepted.

In addition to the initial sales charge paid to your financial advisor and distributor, a 0.25% annual commission is paid to your financial advisor over the life of your investment out of your Fund assets as a 12b-1 fee.

The Fund's distributor may also pay additional compensation to financial advisors as an incentive for promoting the sale of shares of funds that it distributes.

Class A shares bought without an initial sales charge in accounts aggregating $1 million to $50 million at the time of purchase are subject to a 1.00% CDSC if the shares are sold within 12 months of the time of purchase. Subsequent Class A share purchases that bring your account value above $1 million (but less than $50 million) are subject to a CDSC if redeemed within 12 months of the date of purchase. The 12-month period begins on the first day of the month in which the purchase was made. The CDSC does not apply to retirement plans purchasing shares through a fee-based program.

For Class A share purchases of $1 million or more, financial advisors receive a cumulative commission from the distributor as follows:

For group retirement plans, financial advisors are eligible to receive a 1.00% commission from the distributor on purchases, including those in amounts less than $1 million, that are coded as commission eligible trades. In determining the commission payable to a financial advisor in the above table, the breakpoint discount policies described in "Reduced Sales Charges (Commissions) for Larger Investments," on page 13, shall apply.

Certain investments in Class A, B and C shares are subject to a CDSC, a sales charge applied at the time you sell your shares. You will pay the CDSC only on shares you sell within a certain amount of time after purchase. The CDSC generally declines each year until there is no charge for selling shares. The CDSC is applied to the net asset value at the time of purchase or sale, whichever is lower. For purposes of calculating the CDSC, the start of the holding period is the first day of the month in which the purchase was made. Shares you purchase with reinvested dividends or other
distributions are not subject to a CDSC. When you place an order to sell shares, the Fund will automatically sell first those shares not subject to a CDSC and then those you have held the longest.

There are two principal ways you may pay a lower sales charge (often referred to as "breakpoint discounts") when purchasing Class A shares of the Fund and other funds in the Columbia family of funds.

Rights of Accumulation The value of eligible accounts maintained by you and each member of your immediate family may be combined with the value of your current purchase to reach a sales charge discount level (according to the chart on the previous page) and to obtain the lower sales charge for your current purchase. To calculate the combined value of the accounts, the Fund will use the shares' current public offering price.

Statement of Intent You also may pay a lower sales charge when purchasing Class A shares by signing a Statement of Intent. By doing so, you would be able to pay the lower sales charge on all purchases made under the Statement of Intent within 13 months. As described in the chart on the previous page, the first breakpoint discount will be applied when total purchases reach $50,000. If your Statement of Intent purchases are not completed within 13 months, you will be charged the applicable sales charge on the amount you had invested to that date. To calculate the total value of your Statement of Intent purchases, the Fund will use the historic cost (i.e. dollars invested) of the shares held in each eligible account. You must retain all records necessary to substantiate historic costs because the Fund and your financial intermediary may not maintain this information.

The types of eligible accounts that may be aggregated to obtain one or both of the breakpoint discounts described above include:

fund shares. An account is not eligible to the extent it holds an institutional class of shares of a Columbia money market fund or Class R shares of any Columbia fund. The value of your investment in a non-institutional class of a Columbia money market fund held in an eligible account may be aggregated with your investments in other funds in the Columbia family of funds to obtain a breakpoint discount through a Right of Accumulation. Non-institutional class investments in Columbia money market funds may also be included in the aggregation for a Statement of Intent for shares that have been charged a commission.

The steps necessary to obtain a breakpoint discount depend on how your account is maintained with the Columbia family of funds. To obtain any of the above breakpoint discounts, you must notify your financial advisor at the time you purchase shares of the existence of each eligible account maintained by you or your immediate family. It is the sole responsibility of your financial advisor to ensure that you receive discounts for which you are eligible, and the Fund is not responsible for a financial advisor's failure to apply the eligible discount to your account. You may be asked by the Fund or your financial advisor for account statements or other records to verify your discount eligibility, including, where applicable, records for accounts opened with a different financial advisor and records of accounts established by members of your immediate family. If you own shares exclusively through an account maintained with the Fund's transfer agent, Columbia Management Services, Inc., you will need to provide the foregoing information to a Columbia Management Services, Inc. representative at the time you purchase shares.

Certain investors, including affiliates of the Fund, broker/dealers and their affiliates, investors in wrap-fee programs through fee-based advisers or certain retirement plans, certain shareholders of funds that were reorganized into other Columbia funds, and investors using the proceeds of redemptions of Fund shares or of certain Bank of America trust or similar accounts may purchase shares at a reduced sales charge or net asset value, which is the value of a fund share excluding any sales charges. CDSCs may also be waived for redemptions under a systematic withdrawal program, in connection with the death or post-purchase disability of a shareholder, certain medical expenses, charitable gifts, involuntary and tax-related redemptions, or when the selling broker/dealer has agreed to waive or return its commission. Restrictions may apply to certain accounts and certain transactions. Further information regarding these discounts may be found in the Fund's SAI, which can be obtained at www.columbiafunds.com or by calling 1-800-345-6611.

Class B shares No new Class B share accounts may be established in the Fund. However, Class B share accounts established prior to February 3, 2006 may nonetheless purchase additional Class B shares at Class B's net asset value, so long as those purchases do not result in the account holding $50,000 or more of Class B shares (including for this purpose the combined value of all eligible accounts maintained by you that would be used to determine eligibility for reduced sales charges on Class A shares). In the event that an existing Class B shareholder seeks to add funds to such an account so that the balance would be in excess of $49,999, the amount in excess may be invested in Class A shares without the imposition of the minimum investment requirement for Class A shares. Such purchases of Class A shares will be subject to the applicable sales load imposed on such purchases. See "Class A Sales Charges" on page 12.

distributor pays your financial advisor an up-front commission on sales of Class B shares as described in the chart below. In addition, Class B shares bear ongoing service and distribution fees that are higher than those borne by Class A shares.

(2) Applied to the net asset value at the time of purchase or sale, whichever is lower.

In addition, from the annual fee of 0.75% of the value of your shares paid out of your Fund assets as Rule 12b-1 fees, the distributor receives 0.50% of the value of the shares for each of the first eight years, or an aggregate 4.00% over eight years, which will offset the commission it paid to your financial advisor. Your financial advisor is paid the remaining 0.25% as an ongoing commission for the life of your investment.

The conversion of Class B shares to Class A shares occurs automatically eight years after purchase.

Class C shares Your purchases of Class C shares are at Class C's net asset value. Although Class C shares have no front-end sales charge, they carry a CDSC of 1.00% that is applied to shares sold within the first year after they are purchased. After holding shares for one year, you may sell them at any time without paying a CDSC. The distributor pays your financial advisor an up-front commission of 1.00% on your purchase of Class C shares, for which the distributor is repaid from the ongoing annual fee of 1.00% of the value of your shares paid out of your Fund assets as Rule 12b-1 fees, which is higher than the Rule 12b-1 fees borne by Class A and Class B shares.

(1) Applied to the net asset value at the time of purchase or sale, whichever is lower.

A 1.00% annual commission is paid to your financial advisor and the distributor over the life of your investment out of your Fund assets.

to a CDSC, you will not be charged a CDSC upon the exchange. However, when you sell the shares acquired through the exchange, the shares sold may be subject to a CDSC, depending upon when you originally purchased the shares you are exchanging. For purposes of computing the CDSC, the length of time you have owned your shares will be computed from the date of your original purchase and the applicable CDSC will be the CDSC of the original fund. Shareholders of Columbia Acorn Funds that qualify to purchase Class A shares at net asset value may exchange their Class A shares for Class Z shares of another fund distributed by Columbia Management Distributors, Inc. (see the SAI for a description of these situations). Unless your account is part of a tax-deferred retirement plan, an exchange is a taxable event, and you may realize a gain or a loss for tax purposes. The Fund may terminate your exchange privilege if the adviser determines that your exchange activity is likely to adversely impact its ability to manage the Fund. See "Fund Policy on Trading of Fund Shares" for the Fund's policy. To exchange by telephone, call 1-800-422-3737. Please have your account and taxpayer identification numbers available when calling.

You also may exchange your shares through the internet. You will be required to accept the terms of an online agreement and to use a password in order to make internet exchanges. The transfer agent has procedures in place to authenticate electronic instructions given via the internet. For more information on internet transactions, or to sign up for internet transactions, please call 1-800-345-6611.

Your financial advisor can help you determine if and when you should sell your shares. You may sell shares of the Fund on any regular business day that the NYSE is open.

When the Fund receives your sales request in "good form," shares will be sold at the next calculated net asset value. "Good form" means that the Fund's transfer agent has all information and documentation it deems necessary to effect your order. For example, when selling shares by letter of instruction, "good form" means (i) your letter has complete instructions, the proper signatures and Medallion Signature Guarantees, (ii) if applicable, you have included any certificates for shares to be sold, and (iii) any other required documents are attached. For additional documents required for sales by corporations, agents, fiduciaries, surviving joint owners and other legal entities, please call 1-800-345-6611. Retirement plan accounts have special requirements; please call 1-800-799-7526 for more information.

The Fund will generally send proceeds from the sale to you within seven days (usually on the next business day after your request is received in "good form"). However, if you purchased your shares by check, the Fund may delay sending the proceeds from the sale of your shares for up to 10 days after your purchase to protect against checks that are returned. No interest will be paid on uncashed redemption checks. Redemption proceeds may be paid in securities rather than cash, under certain circumstances. For more information, see the paragraph "Non-Cash Redemptions" under the section "How to Sell Shares" in the SAI.

During any 90-day period for any one shareholder, the Fund is obligated to redeem shares solely in cash up to the lesser of $250,000 or 1% of the Fund's net assets. Redemptions in excess of these limits will normally be paid in cash, but may be paid wholly or partly by an in-kind distribution of securities.

The interests of the Fund's long-term shareholders may be adversely affected by certain short-term trading activity by Fund shareholders. Such short-term trading activity, when excessive, has the potential to interfere with efficient portfolio management, generate transaction and other costs, dilute the value of Fund shares held by long-term shareholders and have other adverse effects on the Fund. This type of excessive short-term trading activity is referred to herein as "market timing." For purposes of this section, the "Columbia Funds" are the Columbia, Columbia Acorn Trust and Wanger Advisors trust families of mutual funds. The Columbia Funds are not intended as vehicles for market timing. The Board of Trustees of the Fund has adopted the policies and procedures set forth below with respect to frequent trading of the Fund's shares.

in any 28-day period, except as noted below with respect to orders received through omnibus accounts, the Fund will reject the shareholder's future purchase orders, including exchange purchase orders, involving any Columbia fund (other than a money market fund). In addition, if the Fund determines that any person, group or account has engaged in any type of market timing activity (independent of the two-round-trip limit), the Fund may, in its discretion, reject future purchase orders by the person, group or account, including exchange purchase orders, involving the same or any other Columbia fund, and also retains the right to modify these market timing policies at any time without prior notice.

The rights of shareholders to redeem shares of the Fund are not affected by any of the limits mentioned above. However, certain funds (other than the Fund) impose a redemption fee on the proceeds of fund shares that are redeemed or exchanged within 60 days of their purchase.

For these purposes, a "round trip" is a purchase by any means into a Columbia fund followed by a redemption, of any amount, by any means out of the same Columbia fund. Under this definition, an exchange into the Fund followed by an exchange out of the Fund is treated as a single round trip. Also for these purposes, where known, accounts under common ownership or control generally will be counted together. Accounts maintained or managed by a common intermediary, such as an adviser, selling agent or trust department, generally will not be considered to be under common ownership or control.

Purchases, redemptions and exchanges made through the Columbia Funds' Automatic Investment Plan, Systematic Withdrawal Plan or similar automated plans are not subject to the two-round-trip limit. Purchases, redemptions and exchanges made by Columbia Thermostat Fund are also not subject to the two round-trip limit. The two-round-trip limit may be modified for, or may not be applied to, accounts held by certain retirement plans to conform to plan limits, considerations relating to the Employee Retirement Income Security Act of 1974 or regulations of the Department of Labor, and for certain asset allocation or wrap programs.

The practices and policies described above are intended to deter and curtail market timing in the Fund. However, there can be no assurance that these policies, individually or collectively, will be totally effective in this regard because of various factors. In particular, a substantial portion of purchase, redemption and exchange orders are received through omnibus accounts. Omnibus accounts, in which shares are held in the name of an intermediary on behalf of multiple beneficial owners, are a common form of holding shares among financial intermediaries and retirement plans. The Fund typically is not able to identify trading by a particular beneficial owner through an omnibus account, which may make it difficult or impossible to determine if a particular account is engaged in market timing. Consequently, there is the risk that the Fund may not be able to identify, deter or curtail certain market timing that occurs in the Fund, which may result in certain shareholders being able to market time the Fund while the shareholders in the Fund bear the burden of such activities.

Certain financial intermediaries (including certain retirement plan service providers whose clients include, among others, various retirement plans sponsored by Bank of America and its affiliates for the benefit of its employees (the "Bank of America retirement plan service providers")) have different policies regarding monitoring and restricting market timing in the underlying beneficial owner accounts that they maintain through an omnibus account that may be more or less restrictive than the Fund practices discussed above. In particular, the Bank of America retirement plan service providers permit the reinstatement of future purchase orders for shares of the Fund following various suspension periods. If an intermediary does not enforce the Fund's policy, the intermediary must either provide data transparency to Columbia Management or Columbia Management must

determine that the intermediary's policy taken in its entirely provides protections to Fund shareholders that are generally commensurate with the Fund's policy.

The Fund seeks to act in a manner that it believes is consistent with the best interests of Fund shareholders in making any judgments regarding market timing. Neither the Fund nor its agents shall be held liable for any loss resulting from rejected purchase orders or exchanges.

How the Fund's Share Price is Determined The price of each class of the Fund's shares is based on its net asset value. The net asset value is determined at the close of regular trading on the NYSE, usually 4:00 p.m. Eastern time, on each business day that the NYSE is open for trading (typically Monday through Friday). Net asset value is not determined on days on which the NYSE is closed for trading.

When you request a transaction, it will be processed at the net asset value per share (plus any applicable sales charges) next determined after your request is received in "good form" by the distributor. In most cases, in order to receive that day's price, the distributor must receive your order before that day's transactions are processed. If you request a transaction through your financial advisor, your financial advisor must receive your order by the close of trading on the NYSE to receive that day's price.

The Fund determines its net asset value for each share class by dividing each class's total net assets by the number of that class's outstanding shares. In determining the net asset value, the Fund must determine the value of each security in its portfolio at the close of each trading day. Because the Fund holds securities that are traded on foreign exchanges, the value of the Fund's securities may change on days when shareholders will not be able to buy or sell Fund shares. This will affect the Fund's net asset value on the day it is next determined. Securities for which market quotations are available are valued each day at the current market value. However, where market quotations are unavailable, or when the adviser believes that subsequent events have made them unreliable, the Fund may use other data to determine the fair value of the securities.

The Fund has retained an independent statistical fair value pricing service to assist in the fair valuation process for securities principally traded in a foreign market in order to adjust for possible changes in value that may occur between the close of the foreign exchange and the time at which Fund shares are priced. The use of an independent fair value pricing service is intended to and may decrease the opportunities for time zone arbitrage transactions. There can be no assurance that the use of an independent fair value pricing service will successfully decrease arbitrage opportunities. If a security is valued at a "fair value," that value may be different from the last quoted market price for the security.

You can find the daily prices of some share classes for the Fund in most major daily newspapers under the heading of "Columbia." You can find daily prices for all share classes by visiting www.columbiafunds.com.

Share Certificates Share certificates are not available for any class of shares offered by the Fund. If you currently hold previously issued share certificates, you will not be able to sell your shares until you have returned your share certificates to the transfer agent.

Dividends, Distributions, and Taxes The Fund has the potential to make the following distributions:

Distribution Options The Fund distributes any dividends in June and December and any capital gains (including short-term capital gains) at least annually. You can choose one of the options listed in the table below for these distributions when you open your account. To change your distribution option call 1-800-345-6611.

If you do not indicate on your application or at the time your account is established your preference for handling distributions, the Fund will automatically reinvest all distributions in additional shares of the Fund.

The Fund may earn income from the securities it holds. The Fund also may realize capital gains or losses on sales of its securities. The Fund intends to distribute substantially all of its net investment income and capital gains to shareholders each year. As a shareholder, you are entitled to a portion of the Fund's income and capital gains based on the number of shares you own at the time these distributions are declared.

Distributions of $10 or less will automatically be reinvested in additional Fund shares. If you elect to receive distributions by check and the check is returned as undeliverable, all subsequent distributions will be reinvested in additional shares of the Fund.

Taxes Unless you are an entity exempt from income taxes or invest under a retirement account, regardless of whether you receive your distributions in cash or reinvest them in additional Fund shares, all Fund distributions are subject to federal income tax. Depending on where you live, distributions also may be subject to state and local income taxes.

The Fund intends to elect to be treated and to qualify each year as a regulated investment company. A regulated investment company is not subject to tax at the corporate level on income and gains from investments that are distributed to shareholders in accordance with applicable law. However, the Fund's failure to qualify as a regulated investment company could result in corporate level taxation, and consequently, a reduction in income available for distribution to shareholders.

"qualified dividend income" (as defined in the Internal Revenue Code) eligible for a reduced rate of tax. Properly designated distributions of long-term capital gains (determined by how long the Fund held the investments that generated them) are generally taxable as such, regardless of how long you have held your Fund shares. You will be provided with information each year regarding the amount of ordinary income and capital gains distributed to you for the previous year and any portion of your distribution which is exempt from state and local taxes. Long-term capital gain rates have been reduced — in general, to 15% with lower rates applying to taxpayers in the 10% and 15% rate brackets — for taxable years beginning before January 1, 2011. Distributions are taxable to you even if they are paid from income or gains earned by the Fund before you made your investment (and thus were included in the price you paid). Your investment in the Fund may have additional personal tax implications. Please consult your tax advisor about foreign, federal, state, local or other applicable tax laws.

In addition to the dividends and capital gains distributions made by the Fund, you may realize a capital gain or loss when selling or exchanging shares of the Fund. Such transactions also may be subject to federal, state and local income tax.

Foreign Income Taxes The Fund may receive investment income from sources within foreign countries, and that income may be subject to foreign income taxes at the source. If the Fund pays non-refundable taxes to foreign governments during the year, the taxes will reduce the Fund's dividends but will still be included in your taxable income.

The Fund is governed by its Board of Trustees. More than 75% of the Fund's Trustees are independent, meaning that they have no affiliation with the adviser or the Funds, apart from their positions as Trustees and the personal investments they may have made as private individuals.

The independent Trustees bring backgrounds in business and the professions and academia to their task of working with the Funds' officers to establish the policies and oversee the activities of the Funds. Among the Trustees' responsibilities are selecting the investment adviser for the Funds; negotiating the advisory agreements; reviewing other contracts; approving investment policies; monitoring Fund operations, performance, and costs; and nominating or selecting new Trustees.

Each Trustee serves a Fund until his or her retirement, resignation, death or removal; or otherwise as specified in the Trust's organizational documents. It is expected that every five years the Trustees will call a meeting of shareholders to elect Trustees. A Trustee must retire at the end of the year in which he or she attains the age of 75. Any Trustee may be removed at a shareholders' meeting by a vote representing two-thirds of the net asset value of all shares of the Funds of the Trust. The mailing address for the Trustees and officers is 227 W. Monroe, Suite 3000, Chicago, Illinois 60606.

For more detailed information on the Board of Trustees, please refer to the SAI.

Columbia Wanger Asset Management, L.P. (CWAM), located at 227 West Monroe Street, Suite 3000, Chicago, Illinois 60606, is the Fund's investment adviser and is responsible for the Fund's management, subject to the oversight of the Fund's Board of Trustees. CWAM and its predecessor have managed mutual funds, including the Fund, since 1992. In its duties as investment adviser, CWAM runs the Fund's day-to-day business, including making investment decisions and placing all orders for the purchase and sale of the Fund's portfolio securities.

CWAM was previously named Liberty Wanger Asset Management, L.P., and its predecessor was named Wanger Asset Management, L.P. (WAM). CWAM is a wholly owned subsidiary of Columbia Management Group, LLC, which is an indirect wholly owned subsidiary of Bank of America Corporation. As of December 31, 2006, CWAM managed more than $32.9 billion in assets.

CWAM's advisory fee for managing the Fund in 2006 was 0.64% of the Fund's average daily net assets. CWAM also received an administrative fee from the Fund in 2006 of 0.04% of the Fund's average daily net assets. A discussion of the factors considered by the Fund's Board of Trustees in approving the Fund's investment advisory contract is included in the Fund's annual report to shareholders for the period ended December 31, 2006.

CWAM uses a team to assist the lead portfolio managers in managing the Fund. Team members share responsibility for providing ideas, information and knowledge in managing the Fund, and each team member has one or more particular areas of expertise. Portfolio managers make strategic decisions and monitor and supervise individual transactions. While certain analysts recommend transactions for approval by the portfolio managers, more seasoned analysts are authorized to buy and sell securities for the Fund within the guidelines set by portfolio managers.

Charles P. McQuaid
Lead portfolio manager
Charles McQuaid is president of the Trust and a member of the Trust's Board of Trustees. He has been president of CWAM since October 2003, chief investment officer of CWAM since October 2003, and was director of research at CWAM and its predecessor from July 1992 through December 2003. Mr. McQuaid has been a member of Columbia Acorn Fund's management team since 1978, co-managed Columbia Acorn Fund from 1995 through September 2003 and has been the Fund's lead portfolio manager since September 2003. He served as CWAM's interim director of international research from October 2003 to December 2004. Mr. McQuaid has been the president of Wanger Advisors Trust since 2003 and a trustee since September 2006. He also manages two domestic separate accounts and two offshore funds.

Robert A. Mohn
Co-portfolio manager
Robert Mohn is a vice president of the Trust. He has been a member of the domestic analytical team at CWAM and its predecessor since August 1992. He has managed Columbia Acorn USA since its inception in 1996, co-managed Columbia Acorn Fund since May 2003, and also manages Wanger U.S. Smaller Companies, a mutual fund underlying variable insurance products, and the U.S. fund of an investment company whose shares are offered only to non-U.S. investors. Mr. Mohn also is a vice president of Wanger Advisors Trust and the director of domestic research for CWAM.

The SAI provides additional information about the portfolio managers' compensation, other accounts managed and ownership of securities in the Fund.

The Trust, CWAM and the trustees of the Trust (collectively, the "Columbia defendants") are named as defendants in class and derivative complaints, which were consolidated in a Multi-District Action in the federal district court for the District of Maryland on February 20, 2004 (the "MDL Action"). These lawsuits contend that defendants permitted certain investors to market time their trades in certain Columbia Acorn Funds. The MDL Action is ongoing. All claims against the Trust and its independent trustees have been dismissed; however, the interested trustees of the Trust are still parties to the MDL Action.

On March 21, 2005, a class action complaint was filed against the Trust in the Superior Court of the Commonwealth of Massachusetts seeking to rescind the contingent deferred sales charges assessed upon redemption of Class B shares of Columbia Acorn Funds due to the alleged market timing of the Columbia Acorn Funds. In addition to the rescission of sales charges, plaintiffs seek recovery of actual damages, attorneys' fees and costs. The case has been transferred to the MDL Action in the federal district court of Maryland.

The Trust and CWAM are also defendants in a class action lawsuit, filed on November 13, 2003 in the Circuit Court of the Third Judicial Circuit, Madison County, Illinois, that alleges, in summary, that the Trust and CWAM exposed shareholders of Columbia Acorn International to trading by market timers by allegedly (a) failing to properly evaluate daily whether a significant event affecting the value of that fund's securities had occurred after foreign markets had closed but before the calculation of the fund's net asset value; (b) failing to implement the fund's portfolio valuation and share pricing policies and procedures; and (c) failing to know and implement applicable rules and regulations concerning the calculation of net asset value ("Fair Valuation Lawsuit"). On April 5, 2005, the United States Court of Appeals for the Seventh Circuit ruled that the plaintiffs' state law claims were preempted under federal law resulting in the dismissal of plaintiffs' complaint. Plaintiffs appealed the Seventh Circuit's ruling to the United States Supreme Court. On June 15, 2006, the Supreme Court reversed the Seventh Circuit's ruling on jurisdictional grounds, and the case ultimately was remanded to the state court.

On April 4, 2006, the plaintiffs and the Columbia defendants named in the MDL Action executed an agreement in principle intended to fully resolve all of the lawsuits consolidated in the MDL Action as well as the Fair Valuation Lawsuit. The court entered a stay after the parties executed the agreement in principle. The settlement is subject to court approval.

motion was granted and the appeal was dismissed. On March 6, 2007, the case was remanded to the District Court, where the parties will seek court approval of the settlement. The terms of the proposed settlement, if approved, will require payments by the funds' adviser and/or its affiliates, including payment of plaintiffs' attorneys' fees and notice to class members. In the event that the settlement is not approved, the plaintiffs may elect to go forward with their appeal and no opinion is expressed regarding the likely outcome or financial impact of such an appeal on any fund.

On or about January 11, 2005, a putative class action lawsuit was filed in federal district court in Massachusetts against the Trust and its trustees, along with Columbia Management Advisors, LLC, the sub-administrator of the Wanger Advisors Funds and the Columbia Acorn Funds. The plaintiffs allege that the defendants failed to submit Proof of Claims in connection with settlements of securities class action lawsuits filed against companies in which the Columbia Acorn Funds held positions. The complaint seeks compensatory and punitive damages, and the disgorgement of all fees paid to the Columbia Management Advisors, LLC. Plaintiffs have voluntarily dismissed the Trust and its independent trustees.

As a result of these matters or any adverse publicity or other developments resulting from them, there may be increased redemptions or reduced sales of Fund shares, which could increase transaction costs or operating expenses, or have other adverse consequences for the Fund.

However, based on currently available information, the Trust believes that the likelihood that these lawsuits will have a material adverse impact on any fund is remote, and CWAM believes that the lawsuits are not likely to materially affect its ability to provide investment management services to the Fund.

The Fund's principal investment strategies and associated risks are described under "The Fund — Principal Investment Strategies" and "The Fund — Principal Investment Risks." This section describes other investments the Fund may make and the risks associated with them. In seeking to achieve its investment goal, the Fund may invest in various types of securities and engage in various investment techniques, which are not the principal focus of the Fund and therefore are not described in this prospectus. These types of securities and investment practices and their associated risks are identified and discussed in the Fund's SAI, which you may obtain free of charge (see back cover). The adviser may elect not to buy any of these securities or use any of these techniques. The Fund may not always achieve its investment goal. Except as otherwise noted, approval by the Fund's shareholders is not required to modify or change the Fund's investment strategies.

The Fund generally invests in entrepreneurially managed smaller- and mid-sized companies that it believes are not as well known by financial analysts and whose position in a niche creates the opportunity for superior earnings-growth potential. CWAM may identify what it believes are important economic, social or technological trends (for example, the growth of outsourcing as a business strategy, or the productivity gains from the increasing use of technology) and try to identify companies it thinks will benefit from these trends.

In making investments for the Fund, CWAM relies primarily on independent, internally generated research to uncover companies that may be less well known than the more popular names. To find these companies, CWAM compares growth potential, financial strength and fundamental value among companies.

CWAM's analysts continually screen companies and make contact with more than 1,000 companies around the globe each year. To accomplish this, CWAM analysts often talk directly to top management, vendors, suppliers and competitors.

In managing the Fund, CWAM tries to maintain lower taxes and transaction costs by investing with a long-term time horizon (at least two to five years). However, securities purchased on a long-term basis may be sold within 12 months after purchase due to changes in the circumstances of a particular company or industry, or changes in general market or economic conditions.

The Fund may enter into a number of derivative strategies, including those that employ futures, options, straddles or similar transactions, to gain or reduce exposure to particular securities or markets. These strategies, commonly referred to as derivatives, involve the use of financial instruments whose values depend on, or are derived from, the value of an underlying security, index or currency. The Fund may use these strategies to adjust the Fund's sensitivity to changes in interest rates, or for other hedging purposes (i.e., attempting to offset a potential loss in one position by establishing an interest in an opposite position). Derivative strategies involve the risk that they may exaggerate a loss, potentially losing more money than the actual cost of the underlying security, or limit a potential gain. Also, with some derivative strategies there is a risk that the other party to the transaction may fail to honor its contract terms, causing a loss to the Fund or that the Fund may not be able to find a suitable derivative transaction counterparty, and thus may be unable to invest in derivatives altogether.

At times, CWAM may determine that adverse market conditions make it desirable to temporarily suspend the Fund's normal investment activities. During such times, the Fund may, but is not required to, invest in cash or high-quality, short-term debt securities, without limit. Taking a temporary defensive position may prevent the Fund from achieving its investment goal.

The financial highlights table is intended to help you understand the Fund's financial performance. Information is shown for the Fund's last five fiscal years, which run from January 1 to December 31. Certain information in the table reflects the financial results for a single Class A, B or C share. The total returns in the table represent the return that you would have earned (or lost) on an investment in the Fund (assuming reinvestment of all dividends and distributions). This information has been derived from the Fund's financial statements, which have been audited for the years ended December 31, 2004, 2005 and 2006 by PricewaterhouseCoopers LLP, an independent registered public accounting firm, whose report, along with the Fund's financial statements, is included in the Fund's annual report. The information for the years ended December 31, 2002 and 2003 is included in the Fund's financial statements that have been audited by another independent registered public accounting firm, whose report expressed an unqualified opinion on those financial statements and highlights. You can request a free annual report containing those financial statements by calling 1-800-345-6611.

(a) Per share data was calculated using average shares outstanding during the period.

(b) Total return at net asset value assuming all distributions reinvested and no initial sales charge or contingent deferred sales charge.

(c) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have
been reduced.

(a) Per share data was calculated using average shares outstanding during the period.

(b) Total return at net asset value assuming all distributions reinvested and no initial sales charge or contingent deferred sales charge.

(c) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have
been reduced.

(a) Per share data was calculated using average shares outstanding during the period.

(b) Total return at net asset value assuming all distributions reinvested and no initial sales charge or contingent deferred sales charge.

(c) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have
been reduced.

The Fund is required to disclose the following supplemental hypothetical investment information, which provides additional information about the effect of the expenses paid by the Fund, including investment advisory fees and other Fund costs, on the Fund's returns over a 10-year period. The tables show the estimated expenses that would be charged on a hypothetical investment of $10,000 in each class of the Fund assuming a 5% return each year, the cumulative return after fees and expenses, and the hypothetical year-end balance after fees and expenses. The tables also assume that all dividends and distributions are reinvested and that Class B shares convert to Class A shares after eight years. The annual expense ratios used for the Fund, which are the same as those stated in the Annual Fund Operating Expenses table, are presented in the tables, and are net of any contractual fee waivers or expense reimbursements for the period of the contractual commitment. Your actual costs may be higher or lower. The tables shown below reflect the maximum initial sales charges for Class A shares but do not reflect any contingent deferred sales charges which may be payable on redemption. If contingent deferred sales charges were reflected, the "Hypothetical Year-End Balance After Fees and Expenses" amounts shown would be lower and the "Annual Fees and Expenses" amounts shown would be higher. As noted previously, the Fund's minimum investment for initial purchases or exchanges is $75,000.

(1) Annual Fees and Expenses are calculated based on the average between the beginning and ending balance for each year. All information is calculated on an annual compounding basis.

Additional information about the Fund's investments is available in the Fund's semiannual and annual reports to shareholders. The annual report contains a discussion of the market conditions and investment strategies that significantly affected the Fund's performance during its last fiscal year.

You may wish to read the SAI for more information on the Fund and the securities in which it invests. The SAI is incorporated into this prospectus by reference, which means that it is considered to be part of this prospectus.

You can get free copies of annual and semiannual reports and the SAI, request other information and discuss your questions about the Fund by visiting www.columbiafunds.com or by writing or calling the Fund's transfer agent at:

Columbia Management Services, Inc.
P.O. Box 8081
Boston, MA 02266-8081
1-800-345-6611

Fund documents can be viewed online or downloaded from the EDGAR database on the Securities and Exchange Commission internet site at www.sec.gov.

You can review and copy information about the Fund, including the SAI, by visiting the following location, and you can obtain copies upon payment of a duplicating fee, by electronic request at the e-mail address publicinfo@sec.gov or by writing the:

Public Reference Room
Securities and Exchange Commission
Washington, DC 20549-0102

Information on the operation of the Public Reference Room may be obtained by calling 1-202-942-8090.

Investment Company Act file number:
Columbia Acorn Trust: 811-01829
• Columbia Acorn Fund

©2007 Columbia Management Distributors, Inc.
One Financial Center, Boston, MA 02111-2621
800.426.3750 www.columbiafunds.com

Only eligible investors may purchase Class Z shares. See "Your Account — Eligible Investors" for more information.

Columbia Acorn Fund (Acorn^® Fund) seeks to provide long-term growth of capital.

Acorn Fund generally invests in the stocks of small- and medium-sized companies. The Fund generally invests in the stocks of companies with market capitalizations of less than $5 billion at the time of initial purchase. As long as a stock continues to meet the Fund's other investment criteria, the Fund may choose to hold a stock even if the issuing company's capitalization grows beyond a $5 billion capitalization.

Acorn Fund believes that these smaller companies, which are not as well known by financial analysts, may offer higher return potential than the stocks of larger companies.

• A stock price the Fund's investment adviser believes is reasonable relative to the assets and earning power of the company.

Additional strategies that are not principal investment strategies and the risks associated with them are described later in this prospectus under "Other Investment Strategies and Risks."

The principal risks of investing in the Fund are described below. There are many circumstances (including additional risks that are not described here) that could prevent the Fund from achieving its investment goal. You may lose money by investing in the Fund.

Because the Fund invests in stocks, the price of its shares — its net asset value (NAV) per share — fluctuates daily in response to changes in the market value of stocks.

An investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

The bar chart below shows the Fund's calendar-year total returns (before taxes) for its Class Z shares. The performance table following the bar chart shows how the Fund's average annual total returns for Class Z shares compare with broad measures of market performance for one year, five years and ten years. The chart and table are intended to illustrate some of the risks of investing in the Fund by showing changes in the Fund's performance from year to year. All returns include the reinvestment of dividends and distributions. As with all mutual funds, past performance (before and after taxes) does not predict the Fund's future performance.

Calendar Year Total Returns show the Fund's Class Z share performance for each of the last ten complete calendar years. Those returns include the effects of Fund expenses.

Average Annual Total Returns are a measure of the Fund's Class Z average performance over the past one-year, five-year and ten-year periods. They include the effects of Fund expenses.

The Fund's returns are compared to the Russell 2500 Index (Russell 2500), the Standard & Poor's 500 Index (S&P 500 Index) and the Russell 2000^® Index (Russell 2000^®). The Russell 2500, the Fund's primary benchmark, is a market-weighted index of the 2500 smallest companies that is formed by taking the 3000 largest U.S. companies and eliminating the largest 500 of those companies. The S&P 500 Index is a broad market-weighted average of 500 widely-held, large capitalization U.S. stocks. The Russell 2000^® is a market-weighted index of the 2000 smallest companies of the 3000 largest U.S. companies based on market capitalization. All third-party trademarks are the property of their owners. Unlike the Fund, indices are not investments, do not incur fees, expenses or taxes, and are not professionally managed.

For period shown in bar chart:
Best quarter: 4th quarter 1999, +21.94%
Worst quarter: 3rd quarter 1998, -19.51%

After tax returns are calculated using the highest historical individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on each investor's own tax situation and may differ from those shown. After-tax returns may not be relevant to investors who hold Fund shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

Expenses are one of several factors to consider before you invest in a mutual fund. The tables below describe the fees and expenses you may pay when you buy, hold and sell shares of the Fund.

Annual Fund Operating Expenses are paid by the Fund. They include management and administration fees and other expenses that generally include, but are not limited to, transfer agency, custody, and legal fees as well as costs related to state registration and printing of Fund documents. The specific fees and expenses that make up the Fund's other expenses will vary from time to time and may include fees or expenses not described here. The Fund will likely incur portfolio transaction costs that are in addition to the total annual fund operating expenses disclosed in the fee table. These transaction costs are made up of all costs that are associated with trading securities for the Fund's portfolio and include, but are not limited to, brokerage commissions and market spreads, as well as potential changes to the price of a security due to the Fund's efforts to purchase or sell it. While certain elements of transaction costs are readily identifiable and quantifiable, other elements that can make up a significant amount of the Fund's transaction costs are not. Higher transaction costs reduce the Fund's returns.

(2) "Other expenses" have been restated to reflect contractual changes to the transfer agency fees paid by the Fund.

Example Expenses help you compare the cost of investing in the Fund to the cost of investing in other mutual funds. It uses the following hypothetical conditions:

† For example only since the minimum initial investment in the Fund is $75,000, subject to certain exceptions described in the section "Eligible Investors" beginning on page 8.

Example Expenses for a $10,000 investment (your actual costs may be higher or lower)

See the Appendix for additional hypothetical investment and expense information.

If you are an eligible investor (described on page 8), when the Fund receives your purchase request in "good form," your shares will be bought at the next calculated price. "Good form" means that the Fund's transfer agent has all information and documentation it deems necessary to effect your order. For example "good form" may mean that you have properly placed your order with Columbia Management Services, Inc. or your financial advisor or the Fund's transfer agent has received your completed application, including all necessary signatures. The Fund reserves the right to refuse a purchase order for any reason, including if the Fund believes that doing so would be in the best interest of the Fund and its shareholders. The USA PATRIOT Act may require us to obtain personal information from you that we will use to verify your identity. If you do not provide the information, we may not be able to open your account. If we are unable to verify your customer information, we reserve the right to close your account or take such other steps as we deem reasonable.

You or your financial advisor may acquire shares of the Fund for your account by exchanging shares you own in a different fund distributed by Columbia Management Distributors, Inc. for shares of the same class or Class A of the Fund at no additional cost. To exchange by telephone, call 1-800-422-3737. Please see "How to Exchange Shares" for more information.

Only certain eligible investors may purchase Class Z shares of the Fund (Eligible Investor), directly or by exchange. Please read the following section for a more detailed description of the eligibility requirements. The Eligible Investors described below are subject to different minimum initial investment requirements.

Eligibility for certain waivers, exemptions or share classes by new or existing investors may not be readily available or accessible through all intermediaries or all types of accounts offered by a broker-dealer, bank, third- party administrator or other financial institution (each commonly referred to as an "intermediary"). Accessibility of these waivers through a particular intermediary also may change at any time. If you believe you are eligible to purchase shares under a specific exemption, but are not permitted by your intermediary to do so, please contact your intermediary. You may be asked to provide information, including account statements and other records, regarding your eligibility.

• Any employee (or family member) of Columbia Wanger Asset Management, L.P.; or

• Any group retirement plan, holding Fund shares in an omnibus manner, for which an intermediary or other entity provides services and is not compensated by the Funds for those services, other than payments for shareholder servicing or sub-accounting performed in place of the transfer agent.

• Any group retirement plan, not holding Fund shares in an omnibus manner, for which an intermediary or other entity provides services and is not compensated by the Fund for those services, other than compensation for shareholder servicing or sub-accounting performed in place of the transfer agent;

• Any shareholder (as well as any family member of a shareholder or person listed on an account registration for any account of the shareholder) of a fund distributed by Columbia Management Distributors, Inc. (CMD) (i) who holds Class Z shares; (ii) who holds shares issued upon conversion of Primary A shares held prior to August 22, 2005; (iii) who holds Class A shares that were obtained by exchange of Class Z shares; or (iv) who holds no-load shares issued upon the merger of a fund distributed by CMD;

• Any trustee or director (or family member of a trustee or director) of any fund distributed by CMD (other than the Columbia Acorn Funds);

• Any employee (or family member of an employee) of Bank of America Corporation or a subsidiary (other than Columbia Wanger Asset Management L.P.);

• Any investor participating in an account offered by an intermediary or other entity that provides services to such an account, is paid an asset-based fee by the investor and is not compensated by the Funds for those services, other than payments for shareholder servicing or sub-accounting performed in place of the Fund's transfer agent (each investor purchasing through an intermediary must independently satisfy the $75,000 minimum investment requirement);

• Any client of Bank of America Corporation or a subsidiary purchasing through an asset management company, trust, fiduciary, retirement plan administration or similar arrangement with Bank of America Corporation or the subsidiary;

• Any person investing all or part of the proceeds of a distribution, rollover or transfer of assets into a Columbia Management Individual Retirement Account from any deferred compensation plan which was a shareholder of any of the funds of Columbia Acorn Trust (formerly named Liberty Acorn Trust) on September 29, 2000, in which the investor was a participant and through which the investor invested in one or more of the funds of Columbia Acorn Trust immediately prior to the distribution, transferor rollover;

• Any investor purchasing through a Columbia Management state tuition plan organized under Section 529 of the Internal Revenue Code;

• Any institutional investor which is a corporation, partnership, trust, foundation, endowment, institution, government entity, or similar organization; which meets the respective qualifications for an accredited investor, as defined under the Securities Act of 1933; or

• Certain financial institutions and intermediaries, such as insurance companies, trust companies, banks, endowments, investment companies or foundations, purchasing shares for its own account, including Bank of America Corporation, its affiliates, or subsidiaries.

The investment minimum is applied at the class level. For group retirement plans, the investment minimum is determined based on the amount of the plan's investment rather than that of its individual participants. The Fund may establish exclusions from the investment minimum from time to time that it deems appropriate and consistent with the interests of shareholders. Please see the SAI for details on these exclusions.

The Fund reserves the right to change the criteria for eligible investors and the investment minimums. For accounts opened prior to December 15, 2003, no minimum investment applies to accounts participating in the automatic investment plan; however, each investment requires a $50 minimum purchase. The Fund also reserves the right to refuse a purchase order for any reason, including if it believes that doing so would be in the best interest of the Fund and its shareholders. Finally, pursuant to the Trust's Agreement and Declaration of Trust, the Fund reserves the right to redeem your shares if your account falls below the minimum investment requirements.

Your purchases of Class Z shares are at net asset value, which is the value of a Class Z share excluding any sales charge. Class Z shares are not subject to an initial sales charge when purchased, or a contingent deferred sales charge when sold.

The Fund also offers three additional classes of shares — Class A, B and C shares are available through a separate prospectus. Each other share class has its own sales charge and expense structure.

Broker-dealers, investment advisers or financial planners selling mutual fund shares may offer their clients more than one class of shares in a fund with different pricing options. This allows you and your financial advisor to choose among different types of sales charges and different levels of ongoing operating expenses, depending on the investment programs your financial advisor offers. Investors should consider carefully any separate transactions and other fees charged by these programs in connection with investing in any available share class before selecting a share class.

Determining which share class is best for you depends on the dollar amount you are investing and the number of years for which you are willing to invest. Based on your personal situation, your financial advisor can help you decide which class of shares makes the most sense for you. In general, anyone who is eligible to purchase Class Z shares, which do not incur Rule 12b-1 fees or sales charges, should do so in preference over other classes.

If you purchase Class Z shares of the Fund through certain intermediaries, they may charge a fee for their services. They may also place limits on your ability to use services the Fund offers. There are no sales charges or limitations if you purchase shares directly from the Fund, except as described in this prospectus.

If an intermediary is an agent or designee of the Fund, orders are processed at the net asset value next calculated after the intermediary receives the order. The intermediary must segregate any orders it receives after the close of regular trading on the NYSE and transmit those orders separately for execution at the net asset value next determined.

You may exchange your shares for Class Z or Class A (only if Class Z is not offered) shares of another fund distributed by CMD at net asset value. Unless your account is part of a tax-deferred retirement plan, an exchange is a taxable event, and you may realize a gain or a loss for tax purposes. The Fund may terminate your exchange privilege if the adviser determines that your exchange activity is likely to adversely impact its ability to manage the Fund. See "Fund Policy on Trading of Fund Shares" for the Fund's policy. To exchange by telephone, call 1-800-422-3737. Please have your account and taxpayer identification numbers available when calling.

You may sell shares of the Fund on any regular business day that the NYSE is open.

When the Fund receives your sales request in "good form," shares will be sold at the next calculated net asset value. "Good form" means the Fund's transfer agent has all information and documentation it deems necessary to effect your order. For example, when selling shares by letter of instruction, "good form" means (i) your letter has complete instructions, the proper signatures and Medallion Signature Guarantees, and (ii) any other required documents are attached. For additional documents required for sales by corporations, agents, fiduciaries, surviving joint owners and other legal entities, please call 1-800-345-6611. Retirement plan accounts have special requirements; please call 1-800-799-7526 for more information.

The Fund will generally send proceeds from the sale to you within seven days (usually on the next business day after your request is received in "good form"). However, if you purchased your shares by check, the Fund may delay sending the proceeds from the sale of your shares for up to 10 days after your purchase to protect against checks that are returned. No interest will be paid on uncashed redemption checks. Redemption proceeds may be paid in securities, rather than cash, under certain circumstances. For more information, see the paragraph "Non-Cash Redemptions" under the section "How to Sell Shares" in the SAI.

The interests of the Fund's long-term shareholders may be adversely affected by certain short-term trading activity by Fund shareholders. Such short-term trading activity, when excessive, has the potential to interfere with efficient portfolio management, generate transaction and other costs, dilute the value of Fund shares held by long-term shareholders and have other adverse effects on the Fund. This type of excessive short-term trading activity is referred to herein as "market timing." For purposes of this section, "Columbia Funds" are the Columbia, Columbia Acorn Trust and Wanger Advisors Trust families of mutual funds. The Columbia Funds are not intended as vehicles for market timing. The Board of Trustees of the Fund has adopted the policies and procedures set forth below with respect to frequent trading of the Fund's shares.

The Fund, directly and through its agents, takes various steps designed to deter and curtail market timing. For example, if the Fund detects that any shareholder has conducted two "round trips" (as defined below) in the Fund in any 28-day period, except as noted below with respect to orders received through omnibus accounts, the Fund will reject the shareholder's future purchase orders, including exchange purchase orders, involving any Columbia fund (other than a money market fund). In addition, if the Fund determines that any person, group or account has engaged in any type of market timing activity (independent of the two-round-trip limit), the Fund may, in its discretion, reject future purchase orders by the person, group or account, including exchange purchase orders, involving the same or any other Columbia fund, and also retains the right to modify these market timing policies at any time without prior notice.

Certain financial intermediaries, including certain retirement plan service providers whose clients include, among others, various retirement plans sponsored by Bank of America and its affiliates for the benefit of its employees (the "Bank of America retirement plan service providers"), have different policies regarding monitoring and restricting market timing in the underlying beneficial owner accounts that they maintain through an omnibus account that may be more or less restrictive than the Fund practices discussed above. In particular, the Bank of America retirement plan service providers permit the reinstatement of future purchase orders for shares of the Fund following various suspension periods. If an intermediary does not enforce the Fund's policy, the intermediary must either provide data transparency to Columbia Management or Columbia Management must determine that the intermediary's policy taken in its entirety provides protections to Fund Shareholders that are generally commensurate with the Fund's policy.

of America. Such increased payments to the other Bank of America affiliate may enable the other Bank of America affiliate to offset credits that it may provide to its customers in order to avoid having such customers pay fees to multiple Bank of America entities in connection with the customer's investment in the Fund.

Payments also may be made to certain financial intermediaries, including other Bank of America affiliates, that provide investor services to retirement plans and other investment programs to compensate financial intermediaries for services they provide to such programs, including, but not limited to, sub-accounting, sub-transfer agency, similar shareholder or participant recordkeeping, shareholder or participant reporting, or shareholder or participant transaction processing. These payments for investor servicing support vary by financial intermediary but generally are not expected, with certain limited exceptions, to exceed 0.45% of the total Fund assets for Class Z shares in the program on an annual basis.

Amounts paid by the Fund's distributor, investment adviser or their affiliates are paid out of the distributor's, investment adviser's or their affiliates' own resources and do not increase the amount paid by you or the Fund. You can find further details about the payments made by the Fund's distributor, investment adviser or their affiliates and the services provided by financial intermediaries as well as a list of the financial intermediaries to which the Fund's distributor, investment adviser or their affiliates have agreed to make marketing support payments in the Fund's SAI. Your financial intermediary may charge you fees or commissions in addition to those disclosed in this prospectus. You can ask your financial intermediary for information about any payments it receives from the Fund's distributor, investment adviser and their affiliates and any services your financial intermediary provides, as well as fees and/or commissions it charges. In addition, depending on the financial arrangement in place at any particular time, a financial intermediary and its financial consultants also may have a financial incentive for recommending a particular fund or share class over others. You should consult with your financial adviser and review carefully any disclosure by the financial intermediary as to compensation received by your financial adviser.

How the Fund's Share Price is Determined The Fund's Class Z share price is based on its net asset value. The net asset value is determined at the close of regular trading on the NYSE, usually 4:00 p.m. Eastern time, on each business day that the NYSE is open for trading (typically Monday through Friday). Net asset value is not determined on days on which the NYSE is closed for trading.

When you request a transaction, it will be processed at the net asset value per share next determined after your request is received in "good form" by the distributor. In most cases, in order to receive that day's price, the Fund must receive your order before that day's transactions are processed. If you request a transaction through your financial advisor, your financial advisor must receive your order by the close of trading on the NYSE to receive that day's price.

The Fund determines its net asset value for its Class Z shares by dividing total net assets attributable to Class Z shares by the number of outstanding Class Z shares. In determining the net asset value, the Fund must determine the value of each security in its portfolio at the close of each trading day. Because the Fund may hold securities that are traded on foreign exchanges, the value of the Fund's securities may change on days when shareholders will not be able to buy or sell Fund shares. This will affect the Fund's net asset value on the day it is next determined. Securities for which market quotations are available are valued each day at the current market value. However, where market quotations are unavailable, or when the adviser believes that subsequent events have made them unreliable, the Fund may use other data to determine the fair value of the securities.

Dividends, Distributions, and Taxes The Fund has the potential to make the following distributions:

In general, any distributions of dividends, interest and short-term capital gains (determined by how long the Fund held the investments that generated them) are taxable as ordinary income, unless such dividends are "qualified dividend income" (as defined in the Internal Revenue Code) eligible for a reduced rate of tax. Properly designated distributions of long-term capital gains (determined by how long the Fund held the investments that generated them) are generally taxable as such, regardless of how long you have held your Fund shares. You will be provided with information each year regarding the amount of ordinary income and capital gains distributed to you for the previous year and any portion of your distribution which is exempt from state and local taxes. Long-term capital gain rates have been reduced – in general, to 15% with lower rates applying to taxpayers in the 10% and 15% rate brackets – for taxable years beginning before January 1, 2011. Distributions are taxable to you even if they are paid from income or gains earned by the Fund before you made your investment (and thus were included in the price you paid). Your investment in the Fund may have additional personal tax implications. Please consult your tax advisor about foreign, federal, state, local or other applicable tax laws.

The independent Trustees bring backgrounds in business and the professions and academia service to their task of working with the Funds' officers to establish the policies and oversee the activities of the Funds. Among the Trustees' responsibilities are selecting the investment adviser for the Funds; negotiating the advisory agreements; reviewing other contracts; approving investment policies; monitoring Fund operations, performance, and costs; and nominating or selecting new Trustees.

Each Trustee serves until his or her retirement, resignation, death or removal; or otherwise as specified in the Trust's organizational documents. It is expected that every five years the Trustees will call a meeting of shareholders to elect Trustees. A Trustee must retire at the end of the year in which he or she attains the age of 75. Any Trustee may be removed at a shareholders' meeting by a vote representing two-thirds of the net asset value of all shares of the Funds of the Trust. The mailing address for the Trustees and officers is 227 W. Monroe, Suite 3000, Chicago, Illinois 60606.

For more detailed information on the Board of Trustees, please refer to the SAI.

CWAM was previously named Liberty Wanger Asset Management, L.P., and its predecessor was named Wanger Asset Management, L.P. (WAM). CWAM is a wholly owned subsidiary of Columbia Management Group, LLC, which in turn is an indirect wholly owned subsidiary of Bank of America Corporation. As of December 31, 2006, CWAM managed more than $32.9 billion in assets.

CWAM uses a team to assist the portfolio managers in managing the Fund. Team members share responsibility for providing ideas, information and knowledge in managing the Fund, and each team member has one or more particular areas of expertise. Portfolio managers make strategic decisions and monitor and supervise individual transactions. While certain analysts recommend transactions for approval by the portfolio managers, more seasoned analysts are authorized to buy and sell securities for the Fund within the guidelines set by portfolio managers.

Charles P. McQuaid
Lead portfolio manager
Charles McQuaid is president of the Trust and a member of the Trust's Board of Trustees. He has been president of CWAM since October 2003, chief investment officer of CWAM since October 2003, and was director of research at CWAM and its predecessor from July 1992 through December 2003. Mr. McQuaid has been a member of Columbia Acorn Fund's management team since 1978, co-managed Columbia Acorn Fund from 1995 through September 2003 and has been the Fund's lead portfolio manager since September 2003. He served as CWAM's interim director of international research from October 2003 to December 2004. Mr. McQuaid has been president of Wanger Advisors Trust since 2003 and a trustee since September 2006. He also manages two domestic separate accounts and two offshore funds.

The SAI provides additional information about the portfolio managers' compensation, other accounts managed and ownership of securities in the Fund.

On March 21, 2005, a class action complaint was filed against the Trust in the Superior Court of the Commonwealth of Massachusetts seeking to rescind the contingent deferred sales charges assessed upon redemption of Class B shares of Columbia Acorn Funds due to the alleged market timing by certain shareholders of the Columbia Acorn Funds. In addition to the rescission of sales charges, plaintiffs seek recovery of actual damages, attorneys' fees and costs. The case has been transferred to the MDL Action in the federal district court of Maryland.

Seventh Circuit's ruling to the United States Supreme Court. On June 15, 2006, the Supreme Court reversed the Seventh Circuit's ruling on jurisdictional grounds, and the case ultimately was remanded to the state court.

In 2004, CWAM and the trustees of the Columbia Acorn Trust were named as defendants in certain purported shareholder class and derivative actions making claims, including claims under the Investment Company and the Investment Advisers Acts of 1940 and state law. Certain Columbia Acorn Funds were named as nominal defendants. The suits allege, inter alia, that the fees and expenses paid by the funds are excessive and that the funds' advisers and their affiliates inappropriately used fund assets to distribute the funds and for other improper purposes. On March 2, 2005, the actions were consolidated in the Massachusetts federal court as In re Columbia Entities Litigation. The plaintiffs filed a consolidated amended complaint on June 9, 2005. On November 30, 2005, the judge dismissed all claims by plaintiffs and entered final judgment in favor of the defendants. The plaintiffs appealed to the United States Court of Appeals for the First Circuit on December 30, 2005. A stipulation and settlement agreement dated January 19, 2007 was filed in the First Circuit on February 14, 2007, with a joint stipulation of dismissal and motion for remand to obtain district court approval of the settlement. The joint motion was granted and the appeal was dismissed. On March 6, 2007, the case was remanded to the District Court, where the parties will seek court approval of the settlement. The terms of the proposed settlement, if approved, will require payments by the funds' adviser and/or its affiliates, including payment of plaintiffs' attorneys' fees and notice to class members. In the event that the settlement is not approved, the plaintiffs may elect to go forward with their appeal and no opinion is expressed regarding the likely outcome or financial impact of such an appeal on any fund.

On or about January 11, 2005, a putative class action lawsuit was filed in federal district court in Massachusetts against the Trust and its trustees, along with Columbia Management Advisors, LLC, the sub-administrator of the Wanger Advisors Funds and the Columbia Acorn Funds. The plaintiffs allege that the defendants failed to submit Proof of Claims in connection with settlements of securities class action lawsuits filed against companies in which the Columbia Acorn Funds held positions. The complaint seeks compensatory and punitive damages, and the disgorgement of all fees paid to Columbia Management Advisors, LLC. Plaintiffs have voluntarily dismissed the Trust and its independent trustees.

The Fund generally invests in entrepreneurially managed smaller- and mid-sized companies that it believes are not as well known by financial analysts and whose position in a niche creates the opportunity for superior earnings growth potential. CWAM may identify what it believes are important economic, social or technological trends (for example, the growth of outsourcing as a business strategy, or the productivity gains from the increasing use of technology) and try to identify companies it thinks will benefit from those trends.

The financial highlights table is intended to help you understand the Fund's Class Z financial performance. Information is shown for the Fund's last five fiscal years, which run from January 1 to December 31. Certain information in the table reflects the financial results for a single Class Z share. The total returns in the table represent the return that you would have earned (or lost) on an investment in the Fund (assuming reinvestment of all dividends and distributions). This information has been derived from the Fund's financial statements, which have been audited for the years ended December 31, 2004, 2005 and 2006 by PricewaterhouseCoopers LLP, an independent registered public accounting firm, whose report, along with the Fund's financial statements, is included in the Fund's annual report. The information for the years ended December 31, 2002 and 2003 is included in the Fund's financial statements that have been audited by another independent registered public accounting firm, whose report expressed an unqualified opinion on those financial statements and highlights. You can request a free annual report containing those financial statements by calling 1-800-345-6611.

(a) Per share data was calculated using average shares outstanding during the period.

(c) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have been reduced.

(1) Annual Fees and Expenses are calculated based on the average between the beginning and ending balance for each year. All information is calculated on an annual compounding basis.

Columbia Management Services, Inc.
P.O. Box 8081
Boston, MA 02266-8081
1-800-345-6611

Fund documents can be viewed online or downloaded from the EDGAR database on the Securities and Exchange Commission internet site at www.sec.gov.

Public Reference Room
Securities and Exchange Commission
Washington, DC 20549-0102

Information on the operation of the Public Reference Room may be obtained by calling 1-202-942-8090.

Investment Company Act file number:
Columbia Acorn Trust: 811-01829
• Columbia Acorn Fund

©2007 Columbia Management Distributors, Inc.
One Financial Center, Boston, MA 02111-2621
800.426.3750 www.columbiafunds.com

Acorn International^® is a trademark owned and registered by Columbia Acorn Trust, Columbia, Columbia Management^® and Columbia Management Services, Inc. are service marks owned and/or registered by Bank of America

Columbia Acorn International (Acorn International^®) seeks to provide long-term growth of capital.

Acorn International invests generally in stocks of non-U.S. small- and medium-sized companies. The Fund generally invests in the stocks of companies based outside the United States with capitalizations of less than $5 billion at the time of initial purchase. As long as a stock continues to meet the Fund's other investment criteria, the Fund may choose to hold a stock even if the issuing company's capitalization grows beyond a $5 billion capitalization.

Acorn International believes that smaller companies, particularly outside the United States, that are not as well known by financial analysts, may offer higher return potential than the stocks of larger companies.

• A stock price the Fund's investment adviser believes is reasonable relative to the assets and earning power of the company.

Acorn International is an international fund and invests the majority (under normal market conditions, at least 75%) of its total assets in the stocks of foreign companies based in developed markets (for example, Japan, Canada and the United Kingdom) and emerging markets (for example, Mexico, Brazil and Korea).

Additional strategies that are not principal investment strategies and the risks associated with them are described later in this prospectus under "Other Investment Strategies and Risks."

Because the Fund invests in stocks, the price of its shares – its net asset value (NAV) per share – fluctuates daily in response to changes in the market value of stocks.

Market timers. Because the Fund invests predominantly in foreign securities, the Fund may be particularly susceptible to market timers. Market timers generally attempt to take advantage of the way the Fund prices its shares by trading based on market information they expect will lead to a change in the Fund's net asset value on the next pricing day. Market timing activity may be disruptive to Fund management and, since a market timer's profits are effectively paid directly out of the Fund's assets, may negatively impact the investment returns of other shareholders. Although the Fund has adopted certain policies and methods intended to identify and to discourage frequent trading based on this strategy, it cannot ensure that all such activity can be identified or terminated.

An investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

The Fund posts its complete portfolio holdings at as of month-end www.columbiafunds.com approximately 30-40 calendar days after such month-end. Additionally, the Fund discloses its top 15 month-end portfolio holdings at www.columbiafunds.com on or about 15 calendar days following such month-end.

The bar chart below shows the Fund's calendar-year total returns (before taxes) for its Class A shares, excluding sales charges. The performance table following the bar chart shows how the Fund's average annual total returns for Class A, B and C shares, including sales charges, compare broad measures of market performance for one year, five years and the life of the share class. The chart and table are intended to illustrate some of the risks of investing in the Fund by showing the changes in the Fund's performance from year to year. All returns include the reinvestment of dividends and distributions. As with all mutual funds, past performance (before and after taxes) does not predict the Fund's future performance.

Calendar Year Total Returns show the Fund's Class A share performance for each completed calendar year since the Class commenced operations. Those returns include the effects of Fund expenses, but not the effects of sales charges paid directly by the shareholder. If sales charges were included, these returns would be lower.

The Fund's returns are compared to the S&P/Citigroup EMI Global ex-U.S. index, the Morgan Stanley Europe, Australasia and Far East Index (EAFE) and the S&P/Citigroup Global ex-U.S. Cap Range $500M - $5B. The S&P/Citigroup EMI Global ex-U.S., the Fund's primary benchmark, is an index of the bottom 20% of institutionally investable capital of developed and emerging countries (after 9/30/1994), selected by the index sponsor, outside the United States. The Morgan Stanley Capital International (MSCI) Europe, Australasia, Far East (EAFE) Index is a free float-adjusted market capitalization index that is designed to measure developed market equity performance, excluding the U.S. and Canada. The S&P/Citigroup Global ex-U.S. Cap Range $500M - $5B is a subset of the broad market selected by the index sponsor representing the small- and mid-cap developed and emerging markets, excluding the United States. Unlike the Fund, indices are not investments, do not incur fees, expenses or taxes, and are not professionally managed.

For the periods shown in bar chart:
Best quarter: 2nd quarter 2003, +21.37%
Worst quarter: 3rd quarter 2002, -19.96%

Average Annual Total Returns — for periods ended December 31, 2006⁽¹⁾

(2) The inception date for Class A, Class B and Class C shares is October 16, 2000.

Sales Charges (Commissions) are paid directly by shareholders to Columbia Management Distributors, Inc., the Fund's distributor, who compensates your financial advisor.

(1) This charge applies only to certain purchases of Class A shares bought without an initial sales charge that are sold within 12 months of purchase.

(3) A redemption fee of 2.00% may be charged on shares that were owned for 60 days or less. For more information, see "Fund Policy on Trading of Fund Shares" below.

(1) The Fund pays an investment advisory fee of 0.77%. The Fund and its adviser have implemented a breakpoint schedule for the Fund's investment advisory fees. The investment advisory fee rates charged to the Fund will decline as Fund assets grow and will continue to be based on a percentage of the Fund's average daily assets. The annual fee schedule for the Fund is as follows: 1.19% for assets up to $100 million; 0.94% for assets in excess of $100 million and up to $500 million; 0.74% for assets in excess of $500 million. In addition to the management fee, the Fund and the other funds of Columbia Acorn Trust (the "Trust") pay the adviser an administrative fee based on the average daily aggregate net assets of the Trust at the following annual rates: 0.05% of net assets up to $8 billion; 0.04% of the next $8 billion; and 0.03% of net assets in excess of $16 billion. Based on the Trust's average daily net assets as of December 31, 2006, the administrative fee would be at a rate of 0.04% and is included in "Other expenses."

(2) The distribution and service (12b-1) fees for Class B shares have been restated to reflect a reduction of the distribution fee rate by 0.10%, effective August 1, 2006.

(3) "Other expenses" have been restated to reflect contractual changes to the transfer agency fees paid by the Fund.

Example Expenses help you compare the cost of investing in the Fund to the cost of investing in other mutual funds. It uses the following hypothetical conditions:

Example Expenses for a $10,000 investment (your actual costs may be higher or lower)

See the Appendix for additional hypothetical investment and expense information.

The Fund offers three classes of shares in this prospectus — Class A, Class B and Class C. Each share class has its own commission and expense structure that affects the investment return of that class. Determining which share class is best for you depends on various factors, including the amount you are investing, how long you expect to hold your investment and your personal situation.

You should consult with your financial advisor before deciding which share class is most appropriate for you. The following considerations should be part of your assessment:

• Class A shares require an up-front commission payment, but pay lower ongoing Rule 12b-1 fees than Class B and Class C shares. Class B and Class C shares have no up-front commission paid by the shareholder but pay higher ongoing commissions (Rule 12b-1 fees), a deferred commission is imposed on a redemption of Class B shares within six years after purchase, and a deferred commission is imposed on a redemption of Class C shares within one year of purchase. The differential between classes will vary depending on the actual investment return for any given period.

• Class B shares pay an ongoing annual commission (Rule 12b-1 fee) of up to 0.75% of the average value of the shares and automatically convert to Class A shares after eight years. A Class B shareholder of the Fund whose account has a value of less than $50,000 (including for this purpose the combined value of all eligible accounts maintained by that shareholder you that would be used to determine eligibility for reduced sales charges on Class A shares) may purchase additional Class B shares of the Fund to increase the account value up to a maximum of $49,999; any additional investment by the shareholder in that Fund will be invested in Class A shares of the Fund, without regard to the normal investment minimum for Class A shares, but will be subject to the applicable Class A shares up-front commission.

• If you invest $1 million or more, you can purchase Class A shares but not Class C shares.

The minimum initial investment for Class A, Class B and Class C shares generally is $2,500. The investment minimum is applied at the class level. For individual retirement plans (including traditional IRAs, Roth IRAs and Coverdell Education Savings Accounts) and non-omnibus group retirement accounts, the minimum initial investment is $1,000. For non-omnibus group retirement plans, the investment minimum is determined based on the plan's investment rather than on the separate investments of its individual participants. For investors establishing an automatic investment plan, the minimum initial investment is $50. There is no minimum initial investment for omnibus accounts or wrap accounts.

The minimum investment for additional purchases of Class A, Class B and Class C shares is $50 for all investors.

The Fund reserves the right to change these investment minimums. The Fund also reserves the right to refuse a purchase order for any reason, including if it believes that doing so would be in the best interest of the Fund and its shareholders. In addition, pursuant to the Trust's Agreement and Declaration of Trust, the Fund reserves the right to redeem your shares if your account falls below the minimum investment restrictions.

Rule 12b-1 Plan The Fund has adopted a plan under Rule 12b-1 under the Investment Company Act that permits it to pay its distributor ongoing sales and marketing fees (commissions) to support the sale and distribution of Class A, B and C shares and certain services provided to you by your financial advisor, and your financial advisor may receive all or a portion of those fees attributable to your shares. The annual fee, as a percentage of the value of the shares, is normally 0.25% for Class A, 0.75% Class B and 1.00% for Class C shares. These fees are paid out of the assets of these share classes. Over time, these fees reduce the return on your investment. Class B shares automatically convert to Class A shares after eight years, eliminating a portion of the Rule 12b-1 fee thereafter.

The Fund's distributor may also pay additional compensation to financial advisors as an incentive for promoting the sale of shares of funds that it distributes.

For Class A share purchases of $1 million or more, financial advisors receive a cumulative commission from the distributor as follows:

Certain investments in Class A, B and C shares are subject to a CDSC, a sales charge applied at the time you sell your shares. You will pay the CDSC only on shares you sell within a certain amount of time after purchase. The CDSC generally declines each year until there is no charge for selling shares. The CDSC is applied to the net asset value at the time of purchase or sale, whichever is lower. For purposes of calculating the CDSC, the start of the holding period is the first day of the month in which the purchase was made. Shares you purchase with reinvested dividends or other distributions are not subject to a CDSC. When you place an order to sell shares, the Fund will automatically sell first those shares not subject to a CDSC and then those you have held the longest.

There are two principal ways you may pay a lower sales charge (often referred to as "breakpoint discounts") when purchasing Class A shares of the Fund and other funds in the Columbia family of funds.

The types of eligible accounts that may be aggregated to obtain one or both of the breakpoint discounts described above include:

For the purposes of obtaining a breakpoint discount, members of your "immediate family" include your spouse, parent, stepparent, legal guardian, child, stepchild, father in-law and mother in-law. Eligible accounts include those registered in the name of your dealer or other financial intermediary through which you own Columbia fund shares. An account is not eligible to the extent it holds an institutional class of shares of a Columbia money

market fund or Class R shares of any Columbia fund. The value of your investment in a non-institutional class of a Columbia money market fund held in an eligible account may be aggregated with your investments in other funds in the Columbia family of funds to obtain a breakpoint discount through a Right of Accumulation. Non-institutional class investments in Columbia money market funds may also be included in the aggregation for a Statement of Intent for shares that have been charged a commission.

Class B shares Your purchases of Class B shares are at Class B's net asset value. Purchases in Class B shares may not result in the account holding $50,000 or more of Class B shares (including for this purpose the combined value of all eligible accounts maintained by you that would be used to determine eligibility for reduced sales charges on Class A shares). In the event that an existing Class B shareholder seeks to add funds to such an account so that the balance would be in excess of $49,999, the amount in excess may be invested in Class A shares without the imposition of the minimum investment requirement for Class A shares. Such purchases of Class A shares will be subject to the applicable sales load imposed on such purchases. See "Class A Sales Charges" on page 12.

Class B shares have no front-end sales charge, but they do carry a CDSC that is imposed only on shares sold within six years of purchase. The CDSC declines over the six years and disappears in the seventh year. The distributor pays your financial advisor an up-front commission on sales of Class B shares as described in the chart below. In addition, Class B shares bear ongoing service and distribution fees that are higher than those borne by Class A shares.

(1) Applied to the net asset value at the time of purchase or sale, whichever is lower.

The conversion of Class B shares to Class A shares occurs automatically eight years after purchase.

(1) Applied to the net asset value at the time of purchase or sale, whichever is lower.

A 1.00% annual commission is paid to your financial advisor and the distributor over the life of your investment out of your Fund assets.

the original fund. Shareholders of Columbia Acorn Funds that qualify to purchase Class A shares at net asset value may exchange their Class A shares for Class Z shares of another fund distributed by Columbia Management Distributors, Inc. (see the SAI for a description of these situations). Unless your account is part of a tax-deferred retirement plan, an exchange is a taxable event, and you may realize a gain or a loss for tax purposes. The Fund may terminate your exchange privilege if the adviser determines that your exchange activity is likely to adversely impact its ability to manage the Fund. See "Fund Policy on Trading of Fund Shares" for the Fund's policy. To exchange by telephone, call 1-800-422-3737. Please have your account and taxpayer identification numbers available when calling.

Your financial advisor can help you determine if and when you should sell your shares. You may sell shares of the Fund on any regular business day that the NYSE is open.

The interests of the Fund's long-term shareholders may be adversely affected by certain short-term trading activity by Fund shareholders. Such short-term trading activity, when excessive, has the potential to interfere with efficient portfolio management, generate transaction and other costs, dilute the value of Fund shares held by long-term shareholders and have other adverse effects on the Fund. This type of excessive short-term trading activity is referred to herein as "market timing." For purposes of this section, The "Columbia Funds are the Columbia, Columbia Acorn Trust and Wanger Advisors Trust families of mutual funds The Columbia Funds are not intended as vehicles for market timing. The Board of Trustees of the Fund has adopted the policies and procedures set forth below with respect to frequent trading of Fund shares.

The Fund, directly and through its agents, takes various steps designed to deter and curtail market timing. For example, if the Fund detects that any shareholder has conducted two "round trips" (as defined below) in the Fund in any 28-day period, except as noted below with respect to orders received through omnibus accounts, the Fund will reject the shareholder's future purchase orders, including exchange purchase orders, involving any Columbia fund (other than a money market fund). In addition, if the Fund determines that any person, group or account has engaged in any type of market timing activity (independent of the two-round-trip limit), the Fund may, in its discretion, reject future purchase orders by the person, group or account, including exchange purchase orders, involving the same or any other Columbia Fund, and also retains the right to modify these market timing policies at any time without prior notice.

The rights of shareholders to redeem shares of the Fund are not affected by any of the limits mentioned above. However, the Fund imposes a redemption fee on the proceeds of fund shares that are redeemed or exchanged within 60 days of their purchase.

For these purposes, a "round trip" is a purchase by any means into a Columbia Fund followed by a redemption, of any amount, by any means out of the same Columbia Fund. Under this definition, an exchange into the Fund followed by an exchange out of the Fund is treated as a single round trip. Also for these purposes, where known, accounts under common ownership or control generally will be counted together. Accounts maintained or managed by a common intermediary, such as an adviser, selling agent or trust department, generally will not be considered to be under common ownership or control.

Purchases, redemptions and exchanges made through the Columbia funds' Automatic Investment Plan, Systematic Withdrawal Plan or similar automated plans are not subject to the two-round-trip limit. Purchases, redemptions and exchanges made by Columbia Thermostat Fund are also not subject to the two-round-trip limit. The two-round-trip limit may be modified for, or may not be applied to, accounts held by certain retirement plans to conform to plan limits, considerations relating to the Employee Retirement Income Security Act of 1974 or regulations of the Department of Labor, and for certain asset allocation or wrap programs.

The practices and policies described above are intended to deter and curtail market timing in the Fund. However, there can be no assurance that these policies, individually or collectively, will be totally effective in this regard because of various factors. In particular, a substantial portion of purchase, redemption and exchange orders are received through omnibus accounts. Omnibus accounts, in which shares are held in the name of an intermediary on behalf of multiple beneficial owners, are a common form of holding shares among financial intermediaries and retirement plans. The Fund typically is not able to identify trading by a particular beneficial owner through an omnibus account, which may make it difficult or impossible to determine if a particular account is engaged in market timing. Consequently, there is the risk that the Fund may not be able to identify, deter or curtail market timing that occurs in the Fund, which may result in certain shareholders being able to market time the Fund while the shareholders in the Fund bear the burden of such activities.

intermediary must either provide data transparency to Columbia Management or Columbia Management must determine that intermediary's policy taken in its entirety provides protections to Fund shareholders that are generally commensurate with the Fund's policy.

The Fund will assess, subject to limited exceptions, a 2.00% redemption fee on the proceeds of Fund shares that are redeemed (either by selling shares or exchanging into another Columbia fund) within 60 days of their purchase. The redemption fee is paid to the Fund.

The redemption fee is imposed on Fund shares redeemed (including redemptions by exchange) within 60 days of purchase. In determining which shares are being redeemed, the Fund generally applies a first-in, first-out approach. For Fund shares acquired by exchange, the holding period prior to the exchange will not be considered in determining whether to assess the redemption fee.

The redemption fee will not be imposed if you qualify for a waiver and the Fund has received proper notification, unless the waiver is automatic as noted below. The Fund will redeem any shares that are eligible for a waiver first.

A Fund shareholder won't pay an otherwise applicable redemption fee on any of the following transactions:

• shares sold following the death or disability (as defined in the tax code) of the shareholder, including a registered joint owner

• shares sold by or distributions from participant directed retirement plans, such as 401(k), 403(b), 457, Keogh, profit sharing, and money purchase pension accounts, where the Fund does not have access to information about the individual participant account activity, except where the Fund has received an indication that the plan administrator is able to assess the redemption fee to the appropriate accounts (automatic)

• shares sold by certain investment funds, including those that Columbia Management Advisors, LCC or its affiliates may manage (automatic)

• shares sold as part of an automatic rebalancing within an asset allocation program or by certain wrap programs where the program sponsor has provided assurances reasonably satisfactory to the Fund that the program is not designed to be a vehicle for market timing

• shares sold by accounts maintained by a financial institution or intermediary where the Fund has received information reasonably satisfactory to the Fund indicating that the financial institution or intermediary maintaining the account is unable for administrative reasons to assess the redemption fee to underlying shareholders

• shares sold by an account which has demonstrated a hardship, such as a medical emergency, as determined in the absolute discretion of the Fund

• shares that are redeemed or exchanged through Columbia funds' Systematic Withdrawal Plan or similar affiliated or unaffiliated automated plans

• lump-sum or other distributions from a qualified corporate or self-employed retirement plan following retirement (or following attainment of age 59¹/₂ in the case of a "key employee" of a "top heavy" plan)

• distributions from an individual retirement account (IRA) or Custodial Account under Section 403(b)(7) of the tax code, following attainment of age 59¹/₂

The Fund also has the discretion to waive the 2.00% redemption fee if the Fund is in jeopardy of failing the 90% income test or losing its registered investment company qualification for tax purposes.

As described above, certain intermediaries do not assess redemption fees to certain categories of redemptions that do not present significant market timing concerns (such as automatic withdrawal plan redemptions). In these situations, the Fund's ability to assess redemption fees is generally limited by the intermediary's policies and, accordingly, no redemption fees will be assessed on such redemptions.

The Fund determines its net asset value for each share class by dividing each class's total net assets by the number of that class's outstanding shares. In determining the net asset value, the Fund must determine the value of each security in its portfolio at the close of each trading day. Because the Fund holds securities that are traded on foreign exchanges, the value of the Fund's securities may change on days when shareholders will not be able to buy or sell Fund shares. This will affect the Fund's net asset value on the day it is next determined. Securities for which market quotations are available are valued each day at the current market value. However, where market quotations are not available, or when the adviser believes that subsequent events have made them unreliable, the Fund may use other data to determine the fair value of the securities.

Dividends, Distributions, and Taxes The Fund has the potential to make the following distributions:

In general, any distributions of dividends, interest and short-term capital gains (determined by how long the Fund held the investments that generated them) are taxable as ordinary income, unless such dividends are "qualified dividend income" (as defined in the Internal Revenue Code) eligible for a reduced rate of tax. Properly designated distributions of long-term capital gains (determined by how long the Fund held the investments that generated them) are generally taxable as such, regardless of how long you have held your Fund shares. You will be provided with information each year regarding the amount of ordinary income and capital gains distributed to you for the previous year and any portion of your distribution which is exempt from state and local taxes. The Fund's investments in foreign securities may be subject to foreign withholding taxes. You may be entitled to claim a credit or deduction with respect to foreign taxes. Long-term capital gain rates have been reduced — in general, to 15% with lower rates applying to taxpayers in the 10% and 15% rate brackets — for taxable years beginning before January 1, 2011. Distributions are taxable to you even if they are paid from income or gains earned by the Fund before you made your investment (and thus were included in the price you paid). Your investment in the Fund may have additional personal tax implications. Please consult your tax advisor about foreign, federal, state, local or other applicable tax laws.

shareholders to elect Trustees. A Trustee must retire at the end of the year in which he or she attains the age of 75. Any Trustee may be removed at a shareholders' meeting by a vote representing two-thirds of the net asset value of all shares of the Funds of the Trust. The mailing address for the Trustees and officers is 227 W. Monroe, Suite 3000, Chicago, Illinois 60606.

For more detailed information on the Board of Trustees, please refer to the SAI.

CWAM was previously named Liberty Wanger Asset Management, L.P., and its predecessor was named Wanger Asset Management, L.P. (WAM). CWAM is a wholly owned subsidiary of Columbia Management Group, LLC, which in turn is an indirect wholly owned subsidiary of Bank of America Corporation. As of December 31, 2006, CWAM managed more than $32.9 billion in assets.

CWAM's advisory fee for managing the Fund in 2006 was 0.77% of the Fund's average daily net assets. CWAM also received an administrative fee from the Fund in 2006 of 0.04% of the Fund's average daily net assets. A discussion of the factors considered by the Fund's Board of Trustees in approving the Fund's investment advisory contract is included in the Fund's annual report to shareholders for the period ended December 31, 2006.

P. Zachary Egan
Co-portfolio manager
P. Zachary Egan is a vice president of the Trust and co-portfolio manager of Columbia Acorn International since May 2003. He has been CWAM's director of international research since December 2004. He has been a member of the international team since 1999.

Louis J. Mendes
Co-portfolio manager
Louis J. Mendes is a vice president of the Trust and co-portfolio manager of Columbia Acorn International since

May 2003. He has been a member of the international team at CWAM since 2001. Mr. Mendes also is a vice president of Wanger Advisors Trust, and a co-portfolio manager of Wanger International Small Cap, an international mutual fund underlying variable insurance products.

The SAI provides additional information about the portfolio managers' compensation, other accounts managed and ownership of securities in the Fund.

On March 21, 2005, a class action complaint was filed against the Trust in the Superior Court of the Commonwealth of Massachusetts seeking to rescind the contingent deferred sales charges assessed upon redemption of Class B shares of Columbia Acorn Funds due to the alleged market timing by certain shareholders of the Columbia Acorn Funds. In addition to the rescission of sales charges, plaintiffs seek recovery of actual damages, attorneys' fees and costs. The case has been transferred to the MDL Action in the federal district court of Maryland.

The Trust and CWAM are also defendants in a class action lawsuit, filed on November 13, 2003 in the Circuit Court of the Third Judicial Circuit, Madison County, Illinois, that alleges, in summary, that the Trust and CWAM exposed shareholders of Columbia Acorn International Fund to trading by market timers by allegedly (a) failing to properly evaluate daily whether a significant event affecting the value of the Fund's securities had occurred after foreign markets had closed but before the calculation of the Fund's net asset value; (b) failing to implement the Fund's portfolio valuation and share pricing policies and procedures; and (c) failing to know and implement applicable rules and regulations concerning the calculation of net asset value ("Fair Valuation Lawsuit"). On April 5, 2005, the United States Court of Appeals for the Seventh Circuit ruled that the plaintiffs' state law claims were preempted under federal law resulting in the dismissal of plaintiffs' complaint. Plaintiffs appealed the Seventh Circuit's ruling to the United States Supreme Court. On June 15, 2006, the Supreme Court reversed the Seventh Circuit's ruling on jurisdictional grounds, and the case ultimately was remanded to the state court.

and settlement agreement dated January 19, 2007 was filed in the First Circuit on February 14, 2007, with a joint stipulation of dismissal and motion for remand to obtain district court approval of the settlement. The joint motion was granted and the appeal was dismissed. On March 6, 2007, the case was remanded to the District Court, where the parties will seek court approval of the settlement. The terms of the proposed settlement, if approved, will require payments by the funds' adviser and/or its affiliates, including payment of plaintiffs' attorneys' fees and notice to class members. In the event that the settlement is not approved, the plaintiffs may elect to go forward with their appeal and no opinion is expressed regarding the likely outcome or financial impact of such an appeal on any fund.

On or about January 11, 2005, a putative class action lawsuit was filed in federal district court in Massachusetts against the Trust and its trustees, along with Columbia Management Advisors, LLC, the sub-administrator of the Wanger Advisors Funds and the Columbia Acorn Funds. The plaintiffs allege that the defendants failed to submit Proof of Claims in connection with settlements of securities class action lawsuits filed against companies in which the Columbia Acorn Funds held positions. The complaint seeks compensatory and punitive damages, and the disgorgement of all fees paid to Columbia Management Advisors, LLC. Plaintiffs have voluntarily dismissed the Trust and its independent trustees.

The Fund's principal investment strategies and associated risks are described under "The Fund — Principal Investment Strategies" and "The Fund — Principal Investment Risks." This section describes other investments the Fund may make and the risks associated with them. In seeking to achieve its investment goal, the Fund may invest in various types of securities and engage in various investment techniques, which are not the principal focus of the Fund and therefore are not described in this prospectus. These types of securities and investment practices and their associated risks and the Fund's fundamental and non-fundamental investment policies are identified and discussed in the Fund's SAI, which you may obtain free of charge (see back cover). The adviser may elect not to buy any of these securities or use any of these techniques. The Fund may not always achieve its investment goal. Except as otherwise noted, approval by the Fund's shareholders is not required to modify or change the Fund's investment strategies.

The Fund generally invests in entrepreneurially managed smaller- and mid-sized non-U.S. companies that it believes are not as well known by financial analysts and whose position in a niche creates the opportunity for superior earnings-growth potential. CWAM may identify what it believes are important economic, social or

technological trends (for example, the growth of outsourcing as a business strategy, or the productivity gains from the increasing use of technology) and try to identify companies it thinks will benefit from these trends.

The Fund may enter into a number of derivative strategies, including those that employ futures, options, straddles or similar transactions to gain or reduce exposure to particular securities or markets. These strategies, commonly referred to as derivatives, involve the use of financial instruments whose values depend on, or are derived from, the value of an underlying security, index or currency. The Fund may use these strategies to adjust the Fund's sensitivity to changes in interest rates, or for other hedging purposes (i.e., attempting to offset a potential loss in one position by establishing an interest in an opposite position). Derivative strategies involve the risk that they may exaggerate a loss, potentially losing more money than the actual cost of the underlying security, or limit a potential gain. Also, with some derivative strategies there is a risk that the other party to the transaction may fail to honor its contract terms, causing a loss to the Fund or that the Fund may not be able to find a suitable derivative transaction counterparty, and thus may be unable to invest in derivatives altogether.

(a) Per share data was calculated using average shares outstanding during the period.

(b) Total return at net asset value assuming all distributions reinvested and no initial sales charge or contingent deferred sales charge.

(c) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have been reduced.

(a) Per share data was calculated using average shares outstanding during the period.

(c) Total return at net asset value assuming all distributions reinvested and no initial sales charge or contingent deferred sales charge.

(d) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have been reduced.

(a) Per share data was calculated using average shares outstanding during the period.

(c) Total return at net asset value assuming all distributions reinvested and no initial sales charge or contingent deferred sales charge.

(d) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have been reduced.

The Fund is required to disclose the following supplemental hypothetical investment information, which provides additional information about the effect of the expenses paid by the Fund, including investment advisory fees and other Fund costs, on the Fund's returns over a 10-year period. The tables show the estimated expenses that would be charged on a hypothetical investment of $10,000 in each class of the Fund assuming a 5% return each year, the cumulative return after fees and expenses, and the hypothetical year-end balance after fees and expenses. The tables also assume that all dividends and distributions are reinvested and that Class B shares convert to Class A shares after eight years. The annual expense ratios used for the Fund, which are the same as those stated in the Annual Fund Operating Expenses table, are presented in the tables, and are net of any contractual fee waivers or expense reimbursements for the period of the contractual commitment. Your actual costs may be higher or lower. The tables shown below reflect the maximum initial sales charge for Class A shares but do not reflect any contingent deferred sales charges which may be payable on redemption. If contingent deferred sales charges were reflected, the "Hypothetical Year-End Balance After Fees and Expenses" amounts shown would be lower and the "Annual Fees and Expenses" amounts shown would be higher.

(1) Annual Fees and Expenses are calculated based on the average between the beginning and ending balance for each year. All information is calculated on an annual compounding basis.

Columbia Management Services, Inc.
P.O. Box 8081
Boston, MA 02266-8081
1-800-345-6611

Fund documents can be viewed online or downloaded from the EDGAR database on the Securities and Exchange Commission internet site at www.sec.gov.

Public Reference Room
Securities and Exchange Commission
Washington, DC 20549-0102

Information on the operation of the Public Reference Room may be obtained by calling 1-202-942-8090.

Investment Company Act file number:
Columbia Acorn Trust: 811-01829
• Columbia Acorn International

©2007 Columbia Management Distributors, Inc.
One Financial Center, Boston, MA 02111-2621
800.426.3750 www.columbiafunds.com

Only eligible investors may purchase Class Z shares. See "Your Account — Eligible Investors" for more information.

Columbia Acorn International (Acorn International^®) seeks to provide long-term growth of capital.

Acorn International generally invests in stocks of non-U.S. small- and medium-sized companies. The Fund generally invests in the stocks of companies based outside the United States with market capitalizations of less than $5 billion at the time of initial purchase. As long as a stock continues to meet the Fund's other investment criteria, the Fund may choose to hold a stock even if the issuing company's capitalization grows beyond a $5 billion capitalization.

• A stock price the Fund's investment adviser believes is reasonable relative to the assets and earning power of the company.

Additional strategies that are not principal investment strategies and the risks associated with them are described later in this prospectus under "Other Investment Strategies and Risks."

Because the Fund invests in stocks, the price of its shares — its net asset value (NAV) per share — fluctuates daily in response to changes in the market value of stocks.

Market timers. Because the Fund invests predominantly in foreign securities, the Fund may be particularly susceptible to market timers. Market timers generally attempt to take advantage of the way the Fund prices its shares by trading based on market information they expect will lead to a change in the Fund's net asset value on the next pricing day. Market timing activity may be disruptive to Fund management and, since a market timer's profits are effectively paid directly out of the Fund's assets, may negatively impact the investment returns of other shareholders. Although the Fund has adopted certain policies and methods intended to identify and discourage frequent trading based on this strategy, it cannot ensure that all such activity can be identified or terminated.

An investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Calendar Year Total Returns show the Fund's Class Z share performance for each of the last ten complete calendar years. Those returns include the effects of Fund expenses.

Average Annual Total Returns are a measure of the Fund's Class Z average performance over the past one-year, five-year and ten-year periods. They include the effects of Fund expenses.

The Fund's returns are compared to the S&P/Citigroup EMI Global ex-U.S. Index, the Morgan Stanley Europe, Australasia and Far East Index (EAFE) and the S&P/Citigroup Global ex-U.S. Cap Range $500M-$5B. The S&P/Citigroup EMI Global ex-U.S., the Fund's primary benchmark, is an index of the bottom 20% of institutionally investable capital of developed and emerging countries (after 9/30/1994), selected by the index sponsor, outside the United States. The Morgan Stanley Capital International (MSCI) Europe, Australasia, Far East (EAFE) Index is a free float-adjusted market capitalization index that is designed to measure developed market equity performance, excluding the U.S. and Canada. The S&P/Citigroup Global ex-U.S. Cap Range $500M-$5B is a subset of the broad market selected by the index sponsor representing the small- and mid-cap developed and emerging markets, excluding the United States. Unlike the Fund, indices are not investments, do not incur fees, expenses or taxes, and are not professionally managed.

(1) The Fund's performance in 1999 was achieved during a period of unusual market conditions.

For the periods shown in bar chart:
Best quarter: 4th quarter 1999, +41.63%
Worst quarter: 3rd quarter 2002, -19.67%

(1) The Fund's performance in 1999 was achieved during a period of unusual market conditions.

Expenses are one of several factors to consider before you invest in a mutual fund. The tables below describe the fees and expenses you may pay when you buy, hold and sell shares of the Fund.

(1) Charged only when selling or exchanging shares you have owned for 60 days or less. For more information, see "Fund Policy on Trading of Fund Shares and Redemption Fees" below.

(2) "Other expenses" have been restated to reflect contractual changes to the transfer agency fees paid by the Fund.

Example Expenses help you compare the cost of investing in the Fund to the cost of investing in other mutual funds. It uses the following hypothetical conditions:

Example Expenses for a $10,000 investment (your actual costs may be higher or lower)

See the Appendix for additional hypothetical investment and expense information.

If you are an eligible investor (described on page 9), when the Fund receives your purchase request in "good form," your shares will be bought at the next calculated price. "Good form" means that the Fund's transfer agent has all information and documentation it deems necessary to effect your order. For example "good form" may mean that you have properly placed your order with Columbia Management Services, Inc. or your financial advisor or the Fund's transfer agent has received your completed application, including all necessary signatures. The Fund reserves the right to refuse a purchase order for any reason, including if the Fund believes that doing so would be in the best interest of the Fund and its shareholders. The USA PATRIOT Act may require us to obtain certain personal information from you that we will use to verify your identity. If you do not provide the information, we may not be able to open your account. If we are unable to verify your customer information, we reserve the right to close your account or take such other steps as we deem reasonable.

Eligibility for certain waivers, exemptions or share classes by new or existing investors may not be readily available or accessible through all intermediaries or all types of accounts offered by a broker-dealer, bank, third-party administrator or other financial institution (each commonly referred to as an "intermediary"). Accessibility of these waivers through a particular intermediary also may change at any time. If you believe you are eligible to purchase shares under a specific exemption, but are not permitted by your intermediary to do so, please contact your intermediary. You may be asked to provide information, including account statements and other records, regarding your eligibility.

exchange of Class Z shares; or (iv) who holds no-load shares issued upon the merger of a fund distributed by CMD;

• Any trustee or director (or family member of a trustee or director) of any fund distributed by CMD;

• Any employee (or family member of an employee) of Bank of America Corporation or a subsidiary;

• Any investor purchasing through a Columbia Management state tuition plan organized under Section 529 of the Internal Revenue Code;

The Fund reserves the right to change the criteria for eligible investors and the investment minimums. No minimum investment applies to accounts participating in the automatic investment plan; however, each investment requires a $50 minimum purchase. The Fund also reserves the right to refuse a purchase order for any reason, including if it believes that doing so would be in the best interest of the Fund and its shareholders. Finally, pursuant to the Trust's Agreement and Declaration of Trust, the Fund reserves the right to redeem your shares if your account falls below the minimum investment requirements.

The Fund also offers three additional classes of shares — Class A, B and C shares are available through a separate prospectus. Each other share class has its own sales charge and expense structure.

Generally, you may exchange your shares for Class Z or Class A (only if Class Z is not offered) shares of another fund distributed by CMD at net asset value. However, if you exchange shares of Columbia Acorn International that you have owned 60 days or less for shares of a fund distributed by CMD that does not have a redemption fee (including Columbia Acorn Fund, Columbia Acorn USA, Columbia Acorn Select and Columbia Thermostat Fund), the Fund will charge you a redemption fee of 2% of the redemption proceeds. Exchanges between Columbia Acorn International and Columbia Acorn International Select (or a fund distributed by CMD that has a redemption fee) will not be subject to the 2% redemption fee. You also may exchange your Class Z shares of any Fund for Class Z shares or, if there are no Class Z shares, Class A shares of certain other funds distributed by CMD at net asset value without a sales charge. Unless your account is part of a tax-deferred retirement plan, an exchange is a taxable event, and you may realize a gain or a loss for tax purposes. The Funds may terminate your exchange privilege if the adviser determines that your exchange activity is likely to adversely impact its ability to manage the Funds. See "Fund Policy on Trading of Fund Shares and Redemption Fees" for the Fund's policy. To exchange by telephone, call 1-800-422-3737. Please have your account and taxpayer identification numbers available when calling.

You may sell shares of the Fund on any regular business day that the NYSE is open.

When the Fund receives your sales request in "good form," shares will be sold at the next calculated net asset value. "Good form" means that the Fund's transfer agent has all information and documentation it deems necessary to effect your order. For example, when selling shares by letter of instruction, "good form" means (i) your letter has complete instructions, the proper signatures and Medallion Signature Guarantees, and (ii) any other required documents are attached. For additional documents required for sales by corporations, agents, fiduciaries, surviving joint owners and other legal entities, please call 1-800-345- 6611. Retirement plan accounts have special requirements; please call 1-800-799-7526 for more information.

The Fund will generally send proceeds from the sale to you within seven days (usually on the next business day after your request is received in "good form"). However, if you purchased your shares by check, the Fund may delay sending the proceeds from the sale of your shares for up to 10 days after your purchase to protect against checks that are returned. No interest will be paid on uncashed redemption checks. Redemption proceeds may be paid in securities, rather than cash, under certain circumstances. For more information, see the paragraph "Non-Cash Redemptions" under the section "How to Sell Shares" in the SAI.

The interests of the Fund's long-term shareholders may be adversely affected by certain short-term trading activity by Fund shareholders. Such short-term trading activity, when excessive, has the potential to interfere with efficient portfolio management, generate transaction and other costs, dilute the value of Fund shares held by long-term shareholders and have other adverse effects on the Fund. This type of excessive short-term trading activity is referred to herein as "market timing." For purposes of this section "Columbia Funds" are the Columbia, Columbia Acorn Trust and Wanger Advisors Trust families of mutual funds. The Columbia Funds are not intended as vehicles for market timing. The Board of Trustees of the Fund has adopted the policies and procedures set forth below with respect to frequent trading of Fund shares.

The rights of shareholders to redeem shares of the Fund are not affected by any of the limits mentioned above. However, the Fund imposes a redemption fee on the proceeds of Fund shares that are redeemed or exchanged within 60 days of their purchase.

managed by a common intermediary, such as an adviser, selling agent or trust department, generally will not be considered to be under common ownership or control.

Certain financial intermediaries, including certain retirement plan service providers whose clients include, among others, various retirement plans sponsored by Bank of America and its affiliates for the benefit of its employees (the "Bank of America retirement plan service providers"), have different policies regarding monitoring and restricting market timing in the underlying beneficial owner accounts that they maintain through an omnibus account that may be more or less restrictive than the Fund practices discussed above. In particular, the Bank of America retirement plan service providers permit the reinstatement of future purchase orders for shares of the Fund following various suspension periods. If an intermediary does not enforce the Fund's policy, the intermediary must either provide data transparency to Columbia Management or Columbia Management must determine that the intermediary's policy taken in its entirety provides protections to Fund shareholders that are generally commensurate with the Fund's policy.

A Fund shareholder won't pay an otherwise applicable redemption fee on any of the following transactions:

• shares sold following the death or disability (as defined in the tax code) of the shareholder, including a registered joint owner

• shares sold by certain investment funds, including those that Columbia Management Advisors, LLC or its affiliates may manage (automatic)

• shares sold by accounts maintained by a financial institution or intermediary where the Fund has received information reasonably satisfactory to the Fund indicating that the financial institution or intermediary is unable for administrative reasons to assess the redemption fee to underlying shareholders

• shares sold by an account which has demonstrated a hardship, such as a medical emergency, as determined in the absolute discretion of the Fund

• shares that are redeemed or exchanged through Columbia funds' Systematic Withdrawal Plan or similar affiliated or unaffiliated automated plans

• distributions from an individual retirement account (IRA) or Custodial Account under Section 403(b)(7) of the tax code, following attainment of age 59¹/₂

While the financial arrangements may vary for each financial intermediary, the support payments to any one financial intermediary are generally expected to be between 0.05% and 0.45% on an annual basis for payments based on average net assets of the Fund attributable to the financial intermediary, and between 0.10% and 0.25% on an annual basis for firms receiving a payment based on gross sales of the Fund attributable to the financial intermediary. The Fund's distributor, investment adviser or their affiliates may make payments in materially larger amounts or on a basis materially different from those described above when dealing with other affiliates of Bank of America. Such increased payments to the other Bank of America affiliate may enable the other Bank of America affiliate to offset credits that it may provide to its customers in order to avoid having such customers pay fees to multiple Bank of America entities in connection with the customer's investment in the Fund.

Payments also may be made to certain financial intermediaries, including other Bank of America affiliates, that provide investor services to retirement plans and other investment programs to compensate financial intermediaries for services they provide to such programs, including, but not limited to, sub-accounting, sub-transfer agency, similar shareholder or participant recordkeeping, shareholder or participant reporting, or shareholder or participant transaction processing. These payments for investor servicing support vary by financial intermediary but generally are not expected, with certain limited exceptions, to exceed 0.45% of the total Fund assets for Class Z shares in the program on an annual basis.

may charge you fees or commissions in addition to those disclosed in this prospectus. You can ask your financial intermediary for information about any payments it receives from the Fund's distributor, investment adviser and their affiliates and any services your financial intermediary provides, as well as fees and/or commissions it charges. In addition, depending on the financial arrangement in place at any particular time, a financial intermediary and its financial consultants also may have a financial incentive for recommending a particular fund or share class over others. You should consult with your financial adviser and review carefully any disclosure by the financial intermediary as to compensation received by your financial adviser.

The Fund determines its net asset value for its Class Z shares by dividing total net assets attributable to Class Z shares by the number of outstanding Class Z shares. In determining the net asset value, the Fund must determine the value of each security in its portfolio at the close of each trading day. Because the Fund holds securities that are traded on foreign exchanges, the value of the Fund's securities may change on days when shareholders will not be able to buy or sell Fund shares. This will affect the Fund's net asset value on the day it is next determined. Securities for which market quotations are available are valued each day at the current market value. However, where market quotations are unavailable, or when the adviser believes that subsequent events have made them unreliable, the Fund may use other data to determine the fair value of the securities.

Dividends, Distributions, and Taxes The Fund has the potential to make the following distributions:

In general, any distributions of dividends, interest and short-term capital gains (determined by how long the Fund held the investments that generated them) are taxable as ordinary income, unless such dividends are "qualified dividend income" (as defined in the Internal Revenue Code) eligible for a reduced rate of tax. Properly designated distributions of long-term capital gains (determined by how long the Fund held the investments that generated them) are generally taxable as such, regardless of how long you have held your Fund shares. You will be provided with information each year regarding the amount of ordinary income and capital gains distributed to you for the previous year and any portion of your distribution which is exempt from state and local taxes. The Fund's investments in foreign securities may be subject to foreign withholding taxes. You may be entitled to claim a credit or deduction with respect to foreign taxes. Long-term capital gain rates have been reduced — in general, to 15% with lower rates applying to taxpayers in the 10% and 15% rate brackets — for taxable years beginning before January 1, 2011. Distributions are taxable to you even if they are paid from income or gains earned by the Fund before you made your investment (and thus were included in the price you paid). Your investment in the Fund may have additional personal tax implications. Please consult your tax advisor about foreign, federal, state, local or other applicable tax laws.

For more detailed information on the Board of Trustees, please refer to the SAI.

CWAM was previously named Liberty Wanger Asset Management, L.P., and its predecessor was named Wanger Asset Management, L.P. (WAM). CWAM is a wholly owned subsidiary of Columbia Management Group, LLC, which in turn is an indirect wholly owned subsidiary of Bank of America Corporation. As of December 31, 2006, CWAM managed more than $32.9 billion in assets.

Louis J. Mendes
Co-portfolio manager
Louis J. Mendes is a vice president of the Trust and co-portfolio manager of Columbia Acorn International since May 2003. He has been a member of the international team at CWAM since 2001. Mr. Mendes also is a vice president of Wanger Advisors Trust, and a co-portfolio manager of Wanger International Small Cap, an international mutual fund underlying variable insurance products.

The Statement of Additional Information provides additional information about the portfolio managers' compensation, other accounts managed and ownership of securities in the Fund.

The Trust, CWAM and the trustees of the Trust (collectively, the "Columbia defendants") are named as defendants in class and derivative complaints, which were consolidated in a Multi-District Action in the federal district court for the District of Maryland on February 20, 2004 (the "MDL Action"). These lawsuits contend that defendants permitted certain investors to market time their trades in certain Columbia Acorn Funds. The MDL Action is ongoing All claims against the Trust and its independent trustees have been dismissed; however, the interested trustees of the Trust are still parties to the MDL Action.

On March 21, 2005, a class action complaint was filed against the Trust in the Superior Court of the Commonwealth of Massachusetts seeking to rescind the contingent deferred sales charges assessed upon redemption of Class B shares of Columbia Acorn Funds due to the alleged market timing by certain shareholders of the Columbia Acorn Funds. In addition to the rescission of sales charges, plaintiffs seek recovery of actual damages, attorneys' fees and costs. The case has been transferred to the MDL Action in the federal district court of Maryland.

The Trust and CWAM are also defendants in a class action lawsuit, filed on November 13, 2003 in the Circuit Court of the Third Judicial Circuit, Madison County, Illinois, that alleges, in summary, that the Trust and CWAM exposed shareholders of Columbia Acorn International to trading by market timers by allegedly (a) failing to properly evaluate daily whether a significant event affecting the value of the fund's securities had occurred after foreign markets had closed but before the calculation of the fund's net asset value; (b) failing to implement the fund's portfolio valuation and share pricing policies and procedures; and (c) failing to know and implement applicable rules and regulations concerning the calculation of net asset value ("Fair Valuation Lawsuit"). On April 5, 2005, the United States Court of Appeals for the Seventh Circuit ruled that the plaintiffs' state law claims were preempted under federal law resulting in the dismissal of plaintiffs' complaint. Plaintiffs appealed the Seventh Circuit's ruling to the United States Supreme Court. On June 15, 2006, the Supreme Court reversed the Seventh Circuit's ruling on jurisdictional grounds, and the case ultimately was remanded to the state court.

On or about January 11, 2005, a putative class action lawsuit was filed in federal district court in Massachusetts against the Trust and its trustees, along with Columbia Management Advisors, LLC, the sub-administrator of the Wanger Advisors Funds and the Columbia Acorn Funds. The plaintiffs allege that the defendants failed to submit Proof of Claims in connection with settlements of securities class action lawsuits filed against companies in which the Columbia Acorn Funds held positions. The complaint seeks compensatory and punitive damages, and the disgorgement of all fees paid to Columbia Management Advisors, LLC. Plaintiffs have voluntarily dismissed the Trust and its independent trustees.

The Fund's principal investment strategies and associated risks are described under "The Fund — Principal Investment Strategies" and "The Fund — Principal Investment Risks." This section describes other investments the Fund may make and the risks associated with them. In seeking to achieve its investment goal, the Fund may invest in various types of securities and engage in various investment techniques which are not the principal focus of the Fund and therefore are not described in this prospectus. These types of securities and investment practices and their associated risks are identified and discussed in the Fund's SAI, which you may obtain free of charge (see back cover). The adviser may elect not to buy any of these securities or use any of these techniques. The Fund may not always achieve its investment goal. Except as otherwise noted, approval by the Fund's shareholders is not required to modify or change the Fund's investment strategies.

In managing the Fund, CWAM tries to maintain lower taxes and transaction costs by investing with a long-term time horizon (at least two to five years). Occasionally, however, securities purchased on a long-term basis may be sold within 12 months after purchase due to changes in the circumstances of a particular company or industry, or changes in general market or economic conditions.

(a) Per share data was calculated using average shares outstanding during the period.

(d) Had the investment adviser and/or transfer agent not waived and/or reimbursed a portion of expenses, total return would have
been reduced.

(1) Annual Fees and Expenses are calculated based on the average between the beginning and ending balance for each year.
All information is calculated on an annual compounding basis.

Columbia Management Services, Inc.
P.O. Box 8081
Boston, MA 02266-8081
1-800-345-6611

Fund documents can be viewed online or downloaded from the EDGAR database on the Securities and Exchange Commission internet site at www.sec.gov.

Public Reference Room
Securities and Exchange Commission
Washington, DC 20549-0102

Information on the operation of the Public Reference Room may be obtained by calling 1-202-942-8090.

Investment Company Act file number:
Columbia Acorn Trust: 811-01829
• Columbia Acorn International

©2007 Columbia Management Distributors, Inc.
One Financial Center, Boston, MA 02111-2621
800.426.3750 www.columbiafunds.com

Acorn USA^® is a trademark owned and registered by Columbia Acorn Trust, Columbia, Columbia Management^® and Columbia Management Services, Inc. are service marks owned and/or registered by Bank of America

Acorn USA invests generally in stocks of small- and medium-sized U.S. companies. The Fund generally invests in the stocks of U.S. companies with capitalizations of less than $5 billion at the time of initial purchase. As long as a stock continues to meet the Fund's other investment criteria, the Fund may choose to hold a stock even if the issuing company's capitalization grows beyond a $5 billion capitalization.

Acorn USA believes that these smaller companies, which are not as well known by financial analysts, may offer higher return potential than the stocks of larger companies.

• A stock price the Fund's investment adviser believes is reasonable relative to the assets and earning power of the company.

Acorn USA generally invests substantially all of its assets in U.S. companies and, under normal circumstances, will invest at least 80% of its net assets (plus any borrowings for investment purposes) in U.S. companies.

Additional strategies that are not principal investment strategies and the risks associated with them are described later in this prospectus under "Other Investment Strategies and Risks."

Because the Fund invest in stocks, the price of its shares — its net asset value (NAV) per share — fluctuates daily in response to changes in the market value of stocks.

An investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

The bar chart below shows the Fund's calendar-year total returns (before taxes) for its Class A shares, excluding sales charges. The performance table following the bar chart shows how the Fund's average annual total returns for Class A, B and C shares, including sales charges, compare with a broad measure of market performance for one year, five years and the life of the share class. The chart and table are intended to illustrate some of the risks of investing in the Fund by showing changes in the Fund's performance from year to year. All returns include the reinvestment of dividends and distributions. As with all mutual funds, past performance (before and after taxes) does not predict the Fund's future performance.

Calendar Year Total Returns show the Fund's Class A share performance for each complete calendar year since the Class commenced operations. Those returns include the effects of Fund expenses, but not the effects of sales charges paid directly by a shareholder. If sales charges were included, these returns would be lower.

Average Annual Total Returns are a measure of the Fund's average performance over the past one year, five years and the life of the share class. The table shows the returns of each share class and includes the effects of both Fund expenses and current sales charges. Class B share returns do not reflect Class A share returns after conversion of Class B shares to Class A shares — see the section "Your Account — Sales Charges (Commissions)."

The Fund's returns are compared to the Russell 2000^® Index (Russell 2000^®). The Russell 2000^® is a market-weighted index of the 2000 smallest companies of the 3000 largest U.S. companies based on market-capitalization. All third-party trademarks are the property of their owners. Unlike the Fund, an index is not an investment, does not incur fees, expenses or taxes, and is not professionally managed.

For the periods shown in bar chart:
Best quarter: 2nd quarter 2003, +23.06%
Worst quarter: 3rd quarter 2002, -21.36%

Average Annual Total Returns — for periods ended December 31, 2006⁽¹⁾

(2) The inception date for Class A, Class B and Class C shares is October 16, 2000.

Sales Charges (Commissions) are paid directly by shareholders to Columbia Management Distributors, Inc., the Fund's distributor, who compensates your financial advisor.

(1) This charge applies only to certain purchases of Class A shares bought without an initial sales charge that are sold within 12 months of purchase.

(1) The fund pays an investment advisory fee of 0.86%. The Fund and its adviser have implemented a breakpoint schedule for the Fund's investment advisory fees. The investment advisory fee rates charged to the Fund will decline as Fund assets grow and will continue to be based on a percentage of the Fund's average daily assets. The annual fee schedule for the Fund is as follows: 0.94% for assets up to $200 million; 0.89% for assets in excess of $200 million and up to $500 million; 0.84% for assets in excess of $500 million and up to $2 billion; and 0.80% for assets in excess of $2 billion and up to $3 billion; and 0.70% for assets in excess of $3 billion. In addition to the management fee, the Fund and the other funds of Columbia Acorn Trust (the "Trust") pay the adviser an administrative fee based on the average daily aggregate net assets of the Trust at the following annual rates: 0.05% of net assets up to $8 billion; 0.04% of the next $8 billion; and 0.03% of net assets in excess of $16 billion. Based on the Trust's average daily net assets as of December 31, 2006, the administrative fee would be at a rate of 0.04% and is included in "Other expenses."

(2) The distribution and service (12b-1) fees for Class B shares have been restated to reflect a reduction of the distribution fee rate by 0.10%, effective August 1, 2006.

(3) "Other expenses" have been restated to reflect contractual changes to the transfer agency fees paid by the Fund.

Example Expenses help you compare the cost of investing in the Fund to the cost of investing in other mutual funds. It uses the following hypothetical conditions:

† For example only, since the minimum initial investment in the Fund is $75,000.

Example Expenses for a $10,000 investment (your actual costs may be higher or lower)

See the Appendix for additional hypothetical investment and expense information.

You should consult with your financial advisor before deciding which share class is most appropriate for you. The following considerations should be part of your assessment:

• Class A shares require an up-front commission payment, but pay lower ongoing Rule 12b-1 fees than Class B and Class C shares. Class B and Class C shares have no up-front commission paid by the shareholder but pay higher ongoing commissions (Rule 12b-1 fees), and a deferred commission is imposed on a redemption of Class B shares within six years after purchase, and a deferred commission is imposed on a redemption of Class C shares within one year of purchase. The differential between classes will vary depending on the actual investment return for any given period.

• If you invest $1 million or more, you can purchase Class A shares but not Class C shares.

The Fund's distributor, investment adviser or their affiliates will make payments, from their own resources, to certain financial intermediaries, including other Bank of America affiliates, for marketing support services. For purposes of this section the term "financial intermediary" includes any broker, dealer, bank, bank trust department, registered investment adviser, financial planner, retirement plan or other third-party administrator and any other institution having a selling, services or any similar agreement with the Fund's distributor, investment adviser or one of their affiliates. These payments are generally based upon one or more of the following factors: average net assets of the mutual funds distributed by the Fund's distributor attributable to that financial intermediary, gross sales of the mutual funds distributed by the Fund's distributor attributable to that financial intermediary, reimbursement of ticket charges (fees that a financial intermediary firm charges its representatives for effecting transactions in fund shares) or a negotiated lump sum payment.

* Mutual fund wrap programs and group retirement plans that invest $75,000 or more in Class A shares of the Fund will not be subject to a sales charge.

The Fund's distributor may also pay additional compensation to financial advisors as an incentive for promoting the sale of shares of funds that it distributes.

Class A shares bought without initial sales charge in accounts aggregating $1 million to $50 million at the time of purchase are subject to a 1.00% CDSC if the shares are sold within 12 months of the time of purchase. Subsequent Class A share purchases that bring your account value above $1 million (but less than $50 million) are subject to a CDSC if redeemed within 12 months of the date of purchase. The 12-month period begins on the first day of the month in which the purchase was made. The CDSC does not apply to retirement purchasing shares through a fee-based program.

For Class A share purchases of $1 million or more, financial advisors receive a cumulative commission from the distributor as follows:

For group retirement plans, financial advisors are eligible to receive a 1.00% commission from the distributor on purchases, including those in amounts of less than $1 million, that are coded as commission eligible trades. In determining the commission payable to a financial advisor in the above table, the breakpoint discount policies described in "Reduced Sales Charges (Commissions) for Larger Investments," on page 13, shall apply.

Certain investments in Class A, B and C shares are subject to a CDSC, a sales charge applied at the time you sell your shares. You will pay the CDSC only on shares you sell within a certain amount of time after purchase. The CDSC generally declines each year until there is no charge for selling shares. The CDSC is applied to the net asset value at the time of purchase or sale, whichever is lower. For purposes of calculating the CDSC, the start of the holding period is the first day of the month in which the purchase was made. Shares you purchase with reinvested dividends or other distributions are not subject to a CDSC. When you place an order to sell shares, the Fund will automatically sell first those shares not subject to a CDSC and then those you have held the longest.

There are two principal ways you may pay a lower sales charge (often referred to as "breakpoint discounts") when purchasing Class A shares of the Fund and other funds in the Columbia family of funds.

The types of eligible accounts that may be aggregated to obtain one or both of the breakpoint discounts described above include:

Certain investors, including affiliates of the Fund, broker/dealers and their affiliates, investors in wrap-fee programs through fee-based advisers or certain retirement plans, certain shareholders of funds that were reorganized into other Columbia funds and investors using the proceeds of redemptions of Fund shares or of certain Bank of America trust or similar accounts, may purchase shares at a reduced sales charge or net asset value, which is the value of a fund share excluding any sales charges. CDSCs may also be waived for redemptions under a systematic withdrawal program, in connection with the death or post-purchase disability of a shareholder, certain medical expenses, charitable gifts, involuntary and tax-related redemptions, or when the selling broker/dealer has agreed to waive or return its commission. Restrictions may apply to certain accounts and certain transactions. Further information regarding these discounts may be found in the Fund's SAI, which can be obtained at www.columbiafunds.com or by calling 1-800-345-6611.

Class B shares Your purchases of Class B shares are at Class B's net asset value. No new Class B share accounts may be established in the Fund. However, Class B share accounts established prior to February 3, 2006 may nonetheless purchase additional Class B shares, so long as those purchases do not result in the account holding $50,000 or more of Class B shares (including for this purpose the combined value of all eligible accounts maintained by you that would be used to determine eligibility for reduced sales charges on Class A shares). In the event that an existing Class B shareholder seeks to add funds to such an account so that the balance would be in excess of $49,999, the amount in excess may be invested in Class A shares without the imposition of the minimum investment requirement for Class A shares. Such purchases of Class A shares will be subject to the applicable sales load imposed on such purchases. See "Class A Sales Charges" on page 12.

your financial advisor an up-front commission on sales of Class B shares as described in the chart below. In addition, Class B shares bear ongoing service and distribution fees that are higher than those borne by Class A shares.

(2) Applied to the net asset value at the time of purchase or sale, whichever is lower.

The conversion of Class B shares to Class A shares occurs automatically eight years after purchase.

(1) Applied to the net asset value at the time of purchase or sale, whichever is lower.

A 1.00% annual commission is paid to your financial advisor and the distributor over the life of your investment out of your Fund assets.

You may exchange your shares for shares of the same share class (and in some cases, certain other classes) of another fund distributed by Columbia Management Distributors, Inc. at net asset value. If your shares are subject to a CDSC, you will not be charged a CDSC upon the exchange. However, when you sell the shares acquired through the exchange, the shares sold may be subject to a CDSC, depending upon when you originally purchased the shares you are exchanging. For purposes of computing the CDSC, the length of time you have owned your shares will be computed from the date of your original purchase and the applicable CDSC will be the CDSC of the original fund. Shareholders of Columbia Acorn Funds that qualify to purchase Class A shares at net asset value may exchange their Class A shares for Class Z shares of another fund distributed by Columbia Management Distributors, Inc. (See the SAI for a description of these situations.) Unless your account is part of a tax-deferred retirement plan, an exchange is a taxable event, and you may realize a gain or a loss for tax purposes. The Fund may terminate your exchange privilege if the adviser determines that your exchange activity is likely to adversely impact its ability to manage the Fund. See "Fund Policy on Trading of Fund Shares" for the Fund's policy. To exchange by telephone, call 1-800-422-3737. Please have your account and taxpayer identification numbers available when calling.

Your financial advisor can help you determine if and when you should sell your shares. You may sell shares of the Fund on any regular business day that the NYSE is open.

The rights of shareholders to redeem shares of the Fund are not affected by any of the limits mentioned above. However, certain funds (other than the Funds) impose a redemption fee on the proceeds of fund shares that are redeemed or exchanged within 60 days of their purchase.

The practices and policies described above are intended to deter and curtail market timing in the Fund. However, there can be no assurance that these policies, individually or collectively, will be totally effective in this regard because of various factors. In particular, a substantial portion of purchase, redemption and exchange orders are received through omnibus accounts. Omnibus accounts, in which shares are held in the name of an intermediary on behalf of multiple beneficial owners, are a common form of holding shares among financial intermediaries and retirement plans. The Fund typically is not able to identify trading by a particular beneficial owner through an omnibus account, which may make it difficult or impossible to determine if a particular account is engaged in market timing. Consequently, there is the risk that the Fund may not be able to identify, deter or curtail certain market timing do anything in response to market timing that occurs in the Fund, which may result in certain shareholders being able to market time the Fund while the shareholders in the Fund bear the burden of such activities.

Certain financial intermediaries (including certain retirement plan service providers whose clients include, among others, various retirement plans sponsored by Bank of America and its affiliates for the benefit of its employees (the "Bank of America retirement plan service providers")) have different policies regarding monitoring and restricting market timing in the underlying beneficial owner accounts that they maintain through an omnibus account that may be more or less restrictive than the Fund practices discussed above. In particular, the Bank of America retirement plan service plan providers permit the reinstatement of future purchase orders for shares of the Fund following various suspension periods. If an intermediary does not enforce the Fund's policy, the intermediary must either provide data transparency to Columbia Management or Columbia Management must

determine that the intermediary's policy taken in its entirety provides protection to fund shareholders that are generally commensurate with the Fund's policy.

Dividends, Distributions, and Taxes The Fund has the potential to make the following distributions:

In general, any distributions of dividends, interest and short-term capital gains (determined by how long the Fund held the investments that generated them) are taxable as ordinary income, unless such dividends are "qualified dividend income" (as defined in the Internal Revenue Code) eligible for a reduced rate of tax. Properly designated distributions of long-term capital gains (determined by how long the Fund held the investments that generated them) are generally taxable as such, regardless of how long you have held your Fund shares. You will be provided with information each year regarding the amount of ordinary income and capital gains distributed to you for the previous year and any portion of your distribution which is exempt from state and local taxes. Long-term capital gain rates have been reduced — in general, to 15% with lower rates applying to taxpayers in the 10% and 15% rate brackets — for taxable years beginning before January 1, 2011. Distributions are taxable to you even if they are paid from income or gains earned by the Fund before you made your investment (and thus were included in the price you paid). Your investment in the Fund may have additional personal tax implications. Please consult your tax advisor about federal, state, local or other applicable tax laws.

For more detailed information on the Board of Trustees, please refer to the SAI.

CWAM was previously named Liberty Wanger Asset Management, L.P. and its predecessor was named Wanger Asset Management, L.P. (WAM). CWAM is a wholly owned subsidiary of Columbia Management Group, LLC, which is an indirect wholly owned subsidiary of Bank of America Corporation. As of December 31, 2006, CWAM managed more than $32.9 billion in assets.

CWAM's advisory fee for managing the Fund in 2006 was 0.86% of the Fund's average daily net assets. CWAM also received an administrative fee from the Fund in 2006 of 0.04% of the Fund's average daily net assets. A discussion of the factors considered by the Fund's Board of Trustees in approving the Fund's investment advisory contract is included in the Fund's annual report to shareholders for the period ended December 31, 2006.

CWAM uses a team to assist the lead portfolio manager in managing the Fund. Team members share responsibility for providing ideas, information and knowledge in managing the Fund, and each team member has one or more particular areas of expertise. The portfolio manager is responsible for making daily investment decisions and utilizing the management team's input and advice when making buy and sell determinations.

Robert A. Mohn
Lead portfolio manager
Robert Mohn is a vice president of the Trust. He has been a member of the domestic analytical team at CWAM and its predecessor since August 1992. He has managed Columbia Acorn USA since its inception in 1996, co-managed Columbia Acorn Fund since May 2003, and also manages Wanger U.S. Smaller Companies, a mutual fund underlying variable insurance products, and the U.S. fund of an investment company whose shares are offered only to non-U.S. investors. Mr. Mohn also is a vice president of Wanger Advisors Trust and the director of domestic research for CWAM.

The SAI provides additional information about the portfolio manager's compensation, other accounts managed and ownership of securities in the Fund.

The Trust, CWAM and the trustees of the Trust (collectively, the "Columbia defendants") are named as defendants in class and derivative complaints, which were consolidated in a Multi-District Action in the federal district court for the District of Maryland on February 20, 2004 (the "MDL Action"). These lawsuits contend that defendants permitted certain investors to market time their trades in certain Columbia Acorn Funds. The MDL Action All claims against the Trust and its independent trustees have been dismissed; however, the interested trustees of the Trust are still parties to the MDL Action.

Funds. In addition to the rescission of sales charges, plaintiffs seek recovery of actual damages, attorneys' fees and costs. The case has been transferred to the MDL Action in the federal district court of Maryland.