497

The interactive data file included as an exhibit to this filing relates to the supplement to the prospectuses for Columbia Acorn Emerging Markets Fund and Columbia Acorn European Fund filed pursuant to Rule 497(e) under the Securities Act of 1933, as amended, on July 7, 2017 (Accession No. 0001193125-17-224100), which is incorporated herein by reference.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	497
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2016
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Central Index Key	dei_EntityCentralIndexKey	0000002110
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Jul. 07, 2017
Document Effective Date	dei_DocumentEffectiveDate	Jul. 07, 2017
Prospectus Date	rr_ProspectusDate	May 01, 2017

XML

May 01, 2017

Supplement dated July 7, 2017

to the Prospectus, the Summary Prospectus and Statement of Additional Information (“SAI”)

dated May 1, 2017, as supplemented, for the following Fund:

Fund

COLUMBIA ACORN TRUST

Columbia Acorn Emerging Markets Fund^SM

Effective immediately, the Fund’s Summary Prospectus and Prospectus are supplemented as follows:

1. The Annual Fund Operating Expenses table that appears under the section heading “Fees and Expenses of the Fund” is removed and replaced with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	1.15%	1.15%	1.15%	1.15%	1.15%	1.15%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses(c)	0.45%	0.45%	0.45%	0.37%	0.32%	0.45%
Acquired fund fees and expenses	0.01%	0.01%	0.01%	0.01%	0.01%	0.01%
Total annual Fund operating expenses(d)	1.86%	2.61%	1.61%	1.53%	1.48%	1.61%
Fee waivers and/or expense reimbursements(e)	(0.30)%	(0.30)%	(0.30)%	(0.33)%	(0.33)%	(0.30)%
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.56%	2.31%	1.31%	1.20%	1.15%	1.31%

^(c)  Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

^(d)  “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

^(e)  Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.55% for Class A shares, 2.30% for Class C shares, 1.30% for Class R4 shares, 1.19% for Class R5 shares, 1.14% for Class Y shares and 1.30% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

2. The Example table and the paragraph that immediately precedes it under the section heading “Fees and Expenses of the Fund” are removed and replaced with the following:

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first and second year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$725	$1,070	$1,469	$2,582
Class C (assuming redemption of all shares at the end of the period)	$334	$753	$1,330	$2,898
Class C (assuming no redemption of shares)	$234	$753	$1,330	$2,898
Class R4 (whether or not shares are redeemed)	$133	$448	$ 818	$1,859
Class R5 (whether or not shares are redeemed)	$122	$417	$770	$1,766
Class Y (whether or not shares are redeemed)	$117	$401	$744	$1,710
Class Z (whether or not shares are redeemed)	$ 133	$448	$818	$1,859

Supplement dated July 7, 2017

to the Prospectus, the Summary Prospectus and Statement of Additional Information (“SAI”) dated May 1, 2017, as supplemented, for the following Fund:

  Fund

  COLUMBIA ACORN TRUST

  Columbia Acorn European Fund^SM

The Fund’s Summary Prospectus and Prospectus are hereby supplemented as follows:

2. Effective immediately, the Annual Fund Operating Expenses table that appears under the section heading “Fees and Expenses of the Fund” is removed and replaced with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	1.19%	1.19%	1.19%	1.19%	1.19%	1.19%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses(c)(d)	0.57%	0.57%	0.57%	0.51%	0.46%	0.57%
Total annual Fund operating expenses	2.01%	2.76%	1.76%	1.70%	1.65%	1.76%
Fee waivers and/or expense reimbursements(e)	(0.56)%	(0.56)%	(0.56)%	(0.57)%	(0.57)%	(0.56)%
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.45%	2.20%	1.20%	1.13%	1.08%	1.20%

^(c)  Other expenses for Class Y shares are based on estimated amounts for the Fund’s current fiscal year.

^(d)  Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

^(e)  Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.45% for Class A shares, 2.20% for Class C shares, 1.20% for Class R4 shares, 1.13% for Class R5 shares, 1.08% for Class Y shares and 1.20% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

3. Effective immediately, the Example table and the paragraph that immediately precedes it under the section heading “Fees and Expenses of the Fund” are removed and replaced with the following:

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first and second year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$714	$1,064	$1,495	$2,691
Class C (assuming redemption of all shares at the end of the period)	$323	$747	$1,357	$3,005
Class C (assuming no redemption of shares)	$223	$747	$1,357	$3,005
Class R4 (whether or not shares are redeemed)	$122	$442	$846	$1,977
Class R5 (whether or not shares are redeemed)	$115	$421	$813	$1,910
Class Y (whether or not shares are redeemed)	$110	$406	$786	$1,856
Class Z (whether or not shares are redeemed)	$122	$442	$846	$1,977

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

COLUMBIA ACORN TRUST

Prospectus Date

rr_ProspectusDate

May 01, 2017

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated July 7, 2017

to the Prospectus, the Summary Prospectus and Statement of Additional Information (“SAI”)

dated May 1, 2017, as supplemented, for the following Fund:

Fund

COLUMBIA ACORN TRUST

Columbia Acorn Emerging Markets Fund^SM

Effective immediately, the Fund’s Summary Prospectus and Prospectus are supplemented as follows:

1. The Annual Fund Operating Expenses table that appears under the section heading “Fees and Expenses of the Fund” is removed and replaced with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	1.15%	1.15%	1.15%	1.15%	1.15%	1.15%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses(c)	0.45%	0.45%	0.45%	0.37%	0.32%	0.45%
Acquired fund fees and expenses	0.01%	0.01%	0.01%	0.01%	0.01%	0.01%
Total annual Fund operating expenses(d)	1.86%	2.61%	1.61%	1.53%	1.48%	1.61%
Fee waivers and/or expense reimbursements(e)	(0.30)%	(0.30)%	(0.30)%	(0.33)%	(0.33)%	(0.30)%
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.56%	2.31%	1.31%	1.20%	1.15%	1.31%

^(c)  Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

^(d)  “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

^(e)  Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.55% for Class A shares, 2.30% for Class C shares, 1.30% for Class R4 shares, 1.19% for Class R5 shares, 1.14% for Class Y shares and 1.30% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

2. The Example table and the paragraph that immediately precedes it under the section heading “Fees and Expenses of the Fund” are removed and replaced with the following:

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first and second year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$725	$1,070	$1,469	$2,582
Class C (assuming redemption of all shares at the end of the period)	$334	$753	$1,330	$2,898
Class C (assuming no redemption of shares)	$234	$753	$1,330	$2,898
Class R4 (whether or not shares are redeemed)	$133	$448	$ 818	$1,859
Class R5 (whether or not shares are redeemed)	$122	$417	$770	$1,766
Class Y (whether or not shares are redeemed)	$117	$401	$744	$1,710
Class Z (whether or not shares are redeemed)	$ 133	$448	$818	$1,859

Supplement dated July 7, 2017

to the Prospectus, the Summary Prospectus and Statement of Additional Information (“SAI”) dated May 1, 2017, as supplemented, for the following Fund:

  Fund

  COLUMBIA ACORN TRUST

  Columbia Acorn European Fund^SM

The Fund’s Summary Prospectus and Prospectus are hereby supplemented as follows:

2. Effective immediately, the Annual Fund Operating Expenses table that appears under the section heading “Fees and Expenses of the Fund” is removed and replaced with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	1.19%	1.19%	1.19%	1.19%	1.19%	1.19%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses(c)(d)	0.57%	0.57%	0.57%	0.51%	0.46%	0.57%
Total annual Fund operating expenses	2.01%	2.76%	1.76%	1.70%	1.65%	1.76%
Fee waivers and/or expense reimbursements(e)	(0.56)%	(0.56)%	(0.56)%	(0.57)%	(0.57)%	(0.56)%
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.45%	2.20%	1.20%	1.13%	1.08%	1.20%

^(c)  Other expenses for Class Y shares are based on estimated amounts for the Fund’s current fiscal year.

^(d)  Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

^(e)  Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.45% for Class A shares, 2.20% for Class C shares, 1.20% for Class R4 shares, 1.13% for Class R5 shares, 1.08% for Class Y shares and 1.20% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

3. Effective immediately, the Example table and the paragraph that immediately precedes it under the section heading “Fees and Expenses of the Fund” are removed and replaced with the following:

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first and second year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$714	$1,064	$1,495	$2,691
Class C (assuming redemption of all shares at the end of the period)	$323	$747	$1,357	$3,005
Class C (assuming no redemption of shares)	$223	$747	$1,357	$3,005
Class R4 (whether or not shares are redeemed)	$122	$442	$846	$1,977
Class R5 (whether or not shares are redeemed)	$115	$421	$813	$1,910
Class Y (whether or not shares are redeemed)	$110	$406	$786	$1,856
Class Z (whether or not shares are redeemed)	$122	$442	$846	$1,977

Columbia Acorn Emerging Markets Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated July 7, 2017

to the Prospectus, the Summary Prospectus and Statement of Additional Information (“SAI”)

dated May 1, 2017, as supplemented, for the following Fund:

Fund

COLUMBIA ACORN TRUST

Columbia Acorn Emerging Markets Fund^SM

Effective immediately, the Fund’s Summary Prospectus and Prospectus are supplemented as follows:

1. The Annual Fund Operating Expenses table that appears under the section heading “Fees and Expenses of the Fund” is removed and replaced with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	1.15%	1.15%	1.15%	1.15%	1.15%	1.15%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses(c)	0.45%	0.45%	0.45%	0.37%	0.32%	0.45%
Acquired fund fees and expenses	0.01%	0.01%	0.01%	0.01%	0.01%	0.01%
Total annual Fund operating expenses(d)	1.86%	2.61%	1.61%	1.53%	1.48%	1.61%
Fee waivers and/or expense reimbursements(e)	(0.30)%	(0.30)%	(0.30)%	(0.33)%	(0.33)%	(0.30)%
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.56%	2.31%	1.31%	1.20%	1.15%	1.31%

^(c)  Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

^(d)  “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

^(e)  Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.55% for Class A shares, 2.30% for Class C shares, 1.30% for Class R4 shares, 1.19% for Class R5 shares, 1.14% for Class Y shares and 1.30% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

2. The Example table and the paragraph that immediately precedes it under the section heading “Fees and Expenses of the Fund” are removed and replaced with the following:

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first and second year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$725	$1,070	$1,469	$2,582
Class C (assuming redemption of all shares at the end of the period)	$334	$753	$1,330	$2,898
Class C (assuming no redemption of shares)	$234	$753	$1,330	$2,898
Class R4 (whether or not shares are redeemed)	$133	$448	$ 818	$1,859
Class R5 (whether or not shares are redeemed)	$122	$417	$770	$1,766
Class Y (whether or not shares are redeemed)	$117	$401	$744	$1,710
Class Z (whether or not shares are redeemed)	$ 133	$448	$818	$1,859

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

June 30, 2019

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]

rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees

“Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

Columbia Acorn Emerging Markets Fund | Class A

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.15%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.45%

^[1]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.86%

^[2]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.30%)

^[3]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.56%

1 year

rr_ExpenseExampleYear01

$ 725

3 years

rr_ExpenseExampleYear03

1,070

5 years

rr_ExpenseExampleYear05

1,469

10 years

rr_ExpenseExampleYear10

2,582

1 year

rr_ExpenseExampleNoRedemptionYear01

725

3 years

rr_ExpenseExampleNoRedemptionYear03

1,070

5 years

rr_ExpenseExampleNoRedemptionYear05

1,469

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 2,582

Columbia Acorn Emerging Markets Fund | Class C

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.15%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

1.00%

Other expenses

rr_OtherExpensesOverAssets

0.45%

^[1]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total annual Fund operating expenses

rr_ExpensesOverAssets

2.61%

^[2]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.30%)

^[3]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

2.31%

1 year

rr_ExpenseExampleYear01

$ 334

3 years

rr_ExpenseExampleYear03

753

5 years

rr_ExpenseExampleYear05

1,330

10 years

rr_ExpenseExampleYear10

2,898

1 year

rr_ExpenseExampleNoRedemptionYear01

234

3 years

rr_ExpenseExampleNoRedemptionYear03

753

5 years

rr_ExpenseExampleNoRedemptionYear05

1,330

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 2,898

Columbia Acorn Emerging Markets Fund | Class R4

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.15%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.45%

^[1]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.61%

^[2]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.30%)

^[3]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.31%

1 year

rr_ExpenseExampleYear01

$ 133

3 years

rr_ExpenseExampleYear03

448

5 years

rr_ExpenseExampleYear05

818

10 years

rr_ExpenseExampleYear10

1,859

1 year

rr_ExpenseExampleNoRedemptionYear01

133

3 years

rr_ExpenseExampleNoRedemptionYear03

448

5 years

rr_ExpenseExampleNoRedemptionYear05

818

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,859

Columbia Acorn Emerging Markets Fund | Class R5

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.15%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.37%

^[1]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.53%

^[2]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.33%)

^[3]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.20%

1 year

rr_ExpenseExampleYear01

$ 122

3 years

rr_ExpenseExampleYear03

417

5 years

rr_ExpenseExampleYear05

770

10 years

rr_ExpenseExampleYear10

1,766

1 year

rr_ExpenseExampleNoRedemptionYear01

122

3 years

rr_ExpenseExampleNoRedemptionYear03

417

5 years

rr_ExpenseExampleNoRedemptionYear05

770

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,766

Columbia Acorn Emerging Markets Fund | Class Y

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.15%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.32%

^[1]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.48%

^[2]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.33%)

^[3]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.15%

1 year

rr_ExpenseExampleYear01

$ 117

3 years

rr_ExpenseExampleYear03

401

5 years

rr_ExpenseExampleYear05

744

10 years

rr_ExpenseExampleYear10

1,710

1 year

rr_ExpenseExampleNoRedemptionYear01

117

3 years

rr_ExpenseExampleNoRedemptionYear03

401

5 years

rr_ExpenseExampleNoRedemptionYear05

744

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,710

Columbia Acorn Emerging Markets Fund | Class Z

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.15%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.45%

^[1]

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.61%

^[2]

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.30%)

^[3]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.31%

1 year

rr_ExpenseExampleYear01

$ 133

3 years

rr_ExpenseExampleYear03

448

5 years

rr_ExpenseExampleYear05

818

10 years

rr_ExpenseExampleYear10

1,859

1 year

rr_ExpenseExampleNoRedemptionYear01

133

3 years

rr_ExpenseExampleNoRedemptionYear03

448

5 years

rr_ExpenseExampleNoRedemptionYear05

818

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,859

Columbia Acorn European Fund

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

cat_SupplementTextBlock

Supplement dated July 7, 2017

to the Prospectus, the Summary Prospectus and Statement of Additional Information (“SAI”) dated May 1, 2017, as supplemented, for the following Fund:

  Fund

  COLUMBIA ACORN TRUST

  Columbia Acorn European Fund^SM

The Fund’s Summary Prospectus and Prospectus are hereby supplemented as follows:

2. Effective immediately, the Annual Fund Operating Expenses table that appears under the section heading “Fees and Expenses of the Fund” is removed and replaced with the following:

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class C	Class R4	Class R5	Class Y	Class Z
Management fees	1.19%	1.19%	1.19%	1.19%	1.19%	1.19%
Distribution and/or service (12b-1) fees	0.25%	1.00%	0.00%	0.00%	0.00%	0.00%
Other expenses(c)(d)	0.57%	0.57%	0.57%	0.51%	0.46%	0.57%
Total annual Fund operating expenses	2.01%	2.76%	1.76%	1.70%	1.65%	1.76%
Fee waivers and/or expense reimbursements(e)	(0.56)%	(0.56)%	(0.56)%	(0.57)%	(0.57)%	(0.56)%
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.45%	2.20%	1.20%	1.13%	1.08%	1.20%

^(c)  Other expenses for Class Y shares are based on estimated amounts for the Fund’s current fiscal year.

^(d)  Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

^(e)  Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.45% for Class A shares, 2.20% for Class C shares, 1.20% for Class R4 shares, 1.13% for Class R5 shares, 1.08% for Class Y shares and 1.20% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

3. Effective immediately, the Example table and the paragraph that immediately precedes it under the section heading “Fees and Expenses of the Fund” are removed and replaced with the following:

Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first and second year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$714	$1,064	$1,495	$2,691
Class C (assuming redemption of all shares at the end of the period)	$323	$747	$1,357	$3,005
Class C (assuming no redemption of shares)	$223	$747	$1,357	$3,005
Class R4 (whether or not shares are redeemed)	$122	$442	$846	$1,977
Class R5 (whether or not shares are redeemed)	$115	$421	$813	$1,910
Class Y (whether or not shares are redeemed)	$110	$406	$786	$1,856
Class Z (whether or not shares are redeemed)	$122	$442	$846	$1,977

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

June 30, 2019

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Other expenses for Class Y shares are based on estimated amounts for the Fund’s current fiscal year.

Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

Columbia Acorn European Fund | Class A

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.19%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.57%

^[1],[4]

Total annual Fund operating expenses

rr_ExpensesOverAssets

2.01%

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.56%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.45%

1 year

rr_ExpenseExampleYear01

$ 714

3 years

rr_ExpenseExampleYear03

1,064

5 years

rr_ExpenseExampleYear05

1,495

10 years

rr_ExpenseExampleYear10

2,691

1 year

rr_ExpenseExampleNoRedemptionYear01

714

3 years

rr_ExpenseExampleNoRedemptionYear03

1,064

5 years

rr_ExpenseExampleNoRedemptionYear05

1,495

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 2,691

Columbia Acorn European Fund | Class C

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.19%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

1.00%

Other expenses

rr_OtherExpensesOverAssets

0.57%

^[1],[4]

Total annual Fund operating expenses

rr_ExpensesOverAssets

2.76%

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.56%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

2.20%

1 year

rr_ExpenseExampleYear01

$ 323

3 years

rr_ExpenseExampleYear03

747

5 years

rr_ExpenseExampleYear05

1,357

10 years

rr_ExpenseExampleYear10

3,005

1 year

rr_ExpenseExampleNoRedemptionYear01

223

3 years

rr_ExpenseExampleNoRedemptionYear03

747

5 years

rr_ExpenseExampleNoRedemptionYear05

1,357

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 3,005

Columbia Acorn European Fund | Class R4

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.19%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.57%

^[1],[4]

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.76%

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.56%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.20%

1 year

rr_ExpenseExampleYear01

$ 122

3 years

rr_ExpenseExampleYear03

442

5 years

rr_ExpenseExampleYear05

846

10 years

rr_ExpenseExampleYear10

1,977

1 year

rr_ExpenseExampleNoRedemptionYear01

122

3 years

rr_ExpenseExampleNoRedemptionYear03

442

5 years

rr_ExpenseExampleNoRedemptionYear05

846

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,977

Columbia Acorn European Fund | Class R5

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.19%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.51%

^[1],[4]

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.70%

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.57%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.13%

1 year

rr_ExpenseExampleYear01

$ 115

3 years

rr_ExpenseExampleYear03

421

5 years

rr_ExpenseExampleYear05

813

10 years

rr_ExpenseExampleYear10

1,910

1 year

rr_ExpenseExampleNoRedemptionYear01

115

3 years

rr_ExpenseExampleNoRedemptionYear03

421

5 years

rr_ExpenseExampleNoRedemptionYear05

813

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,910

Columbia Acorn European Fund | Class Y

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.19%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.46%

^[1],[4]

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.65%

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.57%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.08%

1 year

rr_ExpenseExampleYear01

$ 110

3 years

rr_ExpenseExampleYear03

406

5 years

rr_ExpenseExampleYear05

786

10 years

rr_ExpenseExampleYear10

1,856

1 year

rr_ExpenseExampleNoRedemptionYear01

110

3 years

rr_ExpenseExampleNoRedemptionYear03

406

5 years

rr_ExpenseExampleNoRedemptionYear05

786

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,856

Columbia Acorn European Fund | Class Z

Risk/Return:

rr_RiskReturnAbstract

Management fees

rr_ManagementFeesOverAssets

1.19%

Distribution and/or service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.57%

^[1],[4]

Total annual Fund operating expenses

rr_ExpensesOverAssets

1.76%

Fee waivers and/or expense reimbursements

rr_FeeWaiverOrReimbursementOverAssets

(0.56%)

^[5]

Total annual Fund operating expenses after fee waivers and/or expense reimbursements

rr_NetExpensesOverAssets

1.20%

1 year

rr_ExpenseExampleYear01

$ 122

3 years

rr_ExpenseExampleYear03

442

5 years

rr_ExpenseExampleYear05

846

10 years

rr_ExpenseExampleYear10

1,977

1 year

rr_ExpenseExampleNoRedemptionYear01

122

3 years

rr_ExpenseExampleNoRedemptionYear03

442

5 years

rr_ExpenseExampleNoRedemptionYear05

846

10 years

rr_ExpenseExampleNoRedemptionYear10

$ 1,977

[1] Other expenses have been restated to reflect changes in the fees paid by the Fund to Columbia Management Investment Services Corp. (the Transfer Agent) and are based on estimated amounts for the Fund’s current fiscal year, taking into consideration the method by which fees paid by the Fund to the Transfer Agent are allocated among the Fund’s Class A, Class C, Class R4 and Class Z shares and are calculated separately for each of Class Y and Class R5 shares.

[2] “Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than “Total Net Expenses” shown in the Financial Highlights section of this prospectus because “Total Net Expenses” do not include acquired fund fees and expenses.

[3] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.55% for Class A shares, 2.30% for Class C shares, 1.30% for Class R4 shares, 1.19% for Class R5 shares, 1.14% for Class Y shares and 1.30% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

[4] Other expenses for Class Y shares are based on estimated amounts for the Fund’s current fiscal year.

[5] Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.45% for Class A shares, 2.20% for Class C shares, 1.20% for Class R4 shares, 1.13% for Class R5 shares, 1.08% for Class Y shares and 1.20% for Class Z shares, through June 30, 2019. This arrangement may only be modified, amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	COLUMBIA ACORN TRUST
Prospectus Date	rr_ProspectusDate	May 01, 2017
Document Creation Date	dei_DocumentCreationDate	Jul. 07, 2017