8-K

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FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): October 22, 2003

AFFILIATED MANAGERS GROUP, INC.
(Exact name of Registrant as specified in charter)

Delaware (State or other jurisdiction of incorporation)	0001-13459 (Commission file number)	043218510 (IRS employer identification no.)
600 Hale Street, Prides Crossing, MA 01965 (Address of principal executive offices) (Zip code)

(617) 747-3300
(Registrant's telephone number, including area code)

Item 7. Exhibits.

        Exhibit 99.1 Registrant's Press Release dated October 22, 2003.

Item 12. Results of Operations and Financial Condition.

        The following information is being furnished under Item 12 of Form 8-K. On October 22, 2003, Registrant issued a press release setting forth its financial and operating results for the most recently completed quarter. A copy of this press release is attached hereto as Exhibit 99.1 and hereby incorporated by reference herein.

        The information in this Current Report on Form 8-K and the Exhibit attached hereto shall not be deemed "filed" for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the "Exchange Act") or otherwise subject to the liabilities of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933 or the Exchange Act, regardless of any general incorporation language in such filing.

SIGNATURES

        Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

	AFFILIATED MANAGERS GROUP, INC.
Date: October 22, 2003	/s/  DARRELL W. CRATE DARRELL W. CRATE Executive Vice President, Chief Financial Officer and Treasurer (and also as Principal Financial and Accounting Officer)

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Item 7. Exhibits.
Item 12. Results of Operations and Financial Condition.

SIGNATURES

EX-99.1

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Exhibit 99.1

Contact: Darrell W. Crate
Affiliated Managers Group, Inc.
(617) 747-3300

AMG Reports Financial and Operating Results for
Third Quarter and Nine Months Ended September 30, 2003

Company Reports EPS of $0.75; Cash EPS of $1.25

Boston, MA, October 22, 2003—Affiliated Managers Group, Inc. (NYSE: AMG) today reported its financial and operating results for the quarter and nine months ended September 30, 2003.

Cash earnings per share ("Cash EPS") for the third quarter of 2003 were $1.25, compared to $1.08 for the third quarter of 2002, while diluted earnings per share for the third quarter of 2003 were $0.75, compared to $0.57 for the same period of 2002. Cash Net Income was $27.5 million for the third quarter of 2003, compared to $24.2 million for the third quarter of 2002. Net Income for the third quarter of 2003 was $16.4 million, compared to $12.8 million for the third quarter of 2002. (Cash EPS and Cash Net Income are defined in the attached tables.)

For the third quarter of 2003, revenue was $128.5 million, compared to $115.3 million for the third quarter of 2002. EBITDA for the third quarter of 2003 was $39.3 million, compared to $32.7 million for the same period of 2002.

For the nine months ended September 30, 2003, Cash Net Income was $76.5 million, while EBITDA was $106.6 million. For the same period, Net Income was $43.2 million, on revenue of $355.4 million. For the nine months ended September 30, 2002, Cash Net Income was $74.6 million, while EBITDA was $105.5 million. For the same period, Net Income was $42.7 million, on revenue of $364.2 million.

Net client cash flows from directly managed assets were $1.1 billion, while inflows of overlay assets were approximately $300 million. These aggregate net client cash flows for the quarter resulted in an increase of approximately $600,000 to AMG's annualized EBITDA. The aggregate assets under management of AMG's affiliated investment management firms at September 30, 2003 were $81.9 billion.

"Our Affiliates produced solid growth in the quarter, driven by strong investment performance and positive net client cash flows," stated William J. Nutt, Chairman and Chief Executive Officer. "With our broad exposure to equity products, AMG benefited from growth in the equity indices during the quarter. In particular, we were pleased to have a material participation in small and mid cap equity products, which performed especially well." Mr. Nutt continued, "In addition, the improvements in the equity market environment have enhanced our prospects for new investments. While it is always difficult to predict the timing of transactions, we are pleased with the progress of our new investment activities."

"AMG's Affiliate Development team executed several growth initiatives during the quarter," stated Sean M. Healey, President and Chief Operating Officer. "Advantage Outsourcing Solutions (AOS), the centralized back office platform we launched earlier this year with our Affiliate Rorer, announced a strategic relationship with SEI Investments to accelerate development. Through this partnership, AOS will continue to independently administer Rorer's back office and those of other AMG Affiliates that choose to use it, while leveraging the resources of Rorer and SEI to expand the platform. In addition, we are broadening the product offerings at our Affiliate, The Managers Funds, by entering into an agreement to acquire the retail mutual fund business of 40--86 Advisors, Inc. (previously Conseco Capital Management, Inc.), a complex of eight funds with total assets under management of approximately $400 million."

AMG is an asset management company with equity investments in a diverse group of mid-sized investment management firms. AMG's strategy is to generate growth through the internal growth of its existing Affiliates, as well as through investments in new Affiliates. AMG's innovative transaction structure allows individual members of each Affiliate's management team to retain or receive significant direct equity ownership in their firm while maintaining operating autonomy. In addition, AMG provides centralized assistance to its Affiliates in strategic matters, marketing, distribution, product development and operations.

Certain matters discussed in this press release may constitute forward-looking statements within the meaning of the federal securities laws. Actual results and the timing of certain events could differ materially from those projected in or contemplated by the forward-looking statements due to a number of factors, including changes in the securities or financial markets or in general economic conditions, the availability of equity and debt financing, competition for acquisitions of interests in investment management firms, the investment performance of our Affiliates and their ability to effectively market their investment strategies, and other risks detailed from time to time in AMG's filings with the Securities and Exchange Commission. Reference is hereby made to the "Cautionary Statements" set forth in the Company's Form 10-K for the year ended December 31, 2002.

Financial Tables Follow

A teleconference will be held with AMG's management at 11:00 a.m. Eastern Time today. Parties interested in listening to the teleconference should dial 1-800-219-6110 (domestic calls) or 1-212-329-1455 (international calls) starting at 10:45 a.m. Eastern Time. Those wishing to listen to the teleconference should dial the appropriate number at least ten minutes before the call begins. The teleconference will also be available for replay approximately one hour after the conclusion of the call. To access the replay, please dial 1-800-405-2236 (domestic calls) or 1-303-590-3000 (international calls), and enter the pass code, 554664. The live call and the replay of the session, and additional financial information referenced during the teleconference, can also be accessed via the Web at www.amg.com.

###

For more information on Affiliated Managers Group, Inc.,
please visit AMG's Web site at www.amg.com.

2

Affiliated Managers Group, Inc.
Financial Highlights
(dollars in thousands, except per share data)

	Three Months Ended 9/30/02		Three Months Ended 9/30/03
Revenue	$	115,258	$	128,465
Net Income	$	12,819	$	16,395
Cash Net Income (A)	$	24,166	$	27,527
EBITDA (B)	$	32,690	$	39,314
Average shares outstanding—diluted		22,301,801		21,967,888
Earnings per share—diluted	$	0.57	$	0.75
Cash earnings per share—diluted (C)	$	1.08	$	1.25

	December 31, 2002		September 30, 2003
Cash and cash equivalents	$	27,708	$	231,093
Senior convertible debt	$	229,023	$	423,186
Mandatory convertible securities	$	230,000	$	230,000
Stockholders' equity	$	571,861	$	594,203

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3

Affiliated Managers Group, Inc.
Financial Highlights
(dollars in thousands, except per share data)

	Nine Months Ended 9/30/02		Nine Months Ended 9/30/03
Revenue	$	364,224	$	355,413
Net Income	$	42,680	$	43,215
Cash Net Income (A)	$	74,561	$	76,489
EBITDA (B)	$	105,536	$	106,608
Average shares outstanding—diluted		22,714,620		21,715,123
Earnings per share—diluted	$	1.88	$	1.99
Cash earnings per share—diluted (C)	$	3.28	$	3.52

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4

Affiliated Managers Group, Inc.
Operating Results
(in millions)

Assets Under Management

Statement of Changes—Quarter to Date

		High Net Worth			Mutual Fund		Institutional		Total
Assets under management, June 30, 2003		$	21,406		$	18,173	$	37,725	$	77,304
	Net client cash flows—directly managed assets		(202	)		341		1,006		1,145
	Net client cash flows—overlay assets		—			—		299		299
	Investment performance		569			1,286		1,338		3,193
Assets under management, September 30, 2003		$	21,773		$	19,800	$	40,368	$	81,941

Statement of Changes—Year to Date

		High Net Worth			Mutual Fund		Institutional		Total
Assets under management, December 31, 2002		$	20,664		$	16,379	$	33,766	$	70,809
	Net client cash flows—directly managed assets		(1,027	)		448		1,604		1,025
	Net client cash flows—overlay assets		—			—		282		282
	Investment performance		2,136			2,973		4,716		9,825
Assets under management, September 30, 2003		$	21,773		$	19,800	$	40,368	$	81,941

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5

Affiliated Managers Group, Inc.
Operating Results
(in thousands)

Financial Results

		Three Months Ended 9/30/02		% of Total	Three Months Ended 9/30/03		% of Total
Revenue
	High Net Worth	$	35,500	31%	$	33,415	26%
	Mutual Fund		40,317	35%		50,094	39%
	Institutional		39,441	34%		44,956	35%
		$	115,258	100%	$	128,465	100%
EBITDA (B)
	High Net Worth	$	10,452	32%	$	10,707	27%
	Mutual Fund		11,735	36%		15,975	41%
	Institutional		10,503	32%		12,632	32%
		$	32,690	100%	$	39,314	100%

		Nine Months Ended 9/30/02		% of Total	Nine Months Ended 9/30/03		% of Total
Revenue
	High Net Worth	$	106,904	29%	$	96,907	27%
	Mutual Fund		120,230	33%		136,286	38%
	Institutional		137,090	38%		122,220	35%
		$	364,224	100%	$	355,413	100%
EBITDA (B)
	High Net Worth	$	32,490	31%	$	30,075	28%
	Mutual Fund		36,199	34%		42,000	40%
	Institutional		36,847	35%		34,533	32%
		$	105,536	100%	$	106,608	100%

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6

Affiliated Managers Group, Inc.
Reconciliation of Performance and Liquidity Measures
(in thousands)

		Three Months Ended 9/30/02			Three Months Ended 9/30/03
Net Income		$	12,819		$	16,395
	Intangible amortization		3,825			4,065
	Intangible-related deferred taxes (E)		5,997			5,950
	Affiliate depreciation (F)		1,525			1,117
Cash Net Income (A)		$	24,166		$	27,527
Cash flow from operations		$	40,348		$	49,348
	Interest expense, net of non-cash items		5,368			4,789
	Current tax provision		2,550			3,372
	Changes in assets and liabilities and other adjustments		(15,576	)		(18,195	)
EBITDA (B)		$	32,690		$	39,314
	Holding company expenses		5,325			5,000
EBITDA Contribution		$	38,015		$	44,314

		Nine Months Ended 9/30/02			Nine Months Ended 9/30/03
Net Income		$	42,680		$	43,215
	Intangible amortization		10,521			12,112
	Intangible-related deferred taxes (E)		17,033			17,849
	Affiliate depreciation (F)		4,327			3,313
Cash Net Income (A)		$	74,561		$	76,489
Cash flow from operations		$	91,901		$	83,034
	Interest expense, net of non-cash items		15,739			14,350
	Current tax provision		11,421			7,114
	Changes in assets and liabilities and other adjustments		(13,525	)		2,110
EBITDA (B)		$	105,536		$	106,608
	Holding company expenses		17,325			14,982
EBITDA Contribution		$	122,861		$	121,590

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7

Affiliated Managers Group, Inc.
Consolidated Statements of Income
(dollars in thousands, except per share data)

		Three Months Ended September 30,						Nine Months Ended September 30,
		2002			2003			2002			2003
Revenue		$	115,258		$	128,465		$	364,224		$	355,413
Operating expenses:
	Compensation and related expenses		41,525			47,054			125,013			126,578
	Amortization of intangible assets		3,825			4,065			10,521			12,112
	Depreciation and other amortization		1,525			1,560			4,327			4,684
	Selling, general and administrative		18,893			21,447			62,561			61,843
	Other operating expenses		4,265			3,741			11,279			11,519
			70,033			77,867			213,701			216,736
Operating income			45,225			50,598			150,523			138,677
Non-operating (income) and expenses:
	Investment and other income		(1,206	)		(3,334	)		(2,598	)		(6,293	)
	Interest expense		5,974			5,901			19,554			17,323
			4,768			2,567			16,956			11,030
Income before minority interest and taxes			40,457			48,031			133,567			127,647
Minority interest (D)			(19,091	)		(20,243	)		(62,433	)		(55,158	)
Income before income taxes			21,366			27,788			71,134			72,489
Income taxes—current			2,550			3,372			11,421			7,114
Income taxes—intangible-related deferred			5,984			5,950			16,898			17,849
Income taxes—other deferred			13			2,071			135			4,311
Net income		$	12,819		$	16,395		$	42,680		$	43,215
Average shares outstanding—basic			21,907,342			21,228,912			22,108,441			21,221,305
Average shares outstanding—diluted			22,301,801			21,967,888			22,714,620			21,715,123
Earnings per share—basic		$	0.59		$	0.77		$	1.93		$	2.04
Earnings per share—diluted		$	0.57		$	0.75		$	1.88		$	1.99

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8

Affiliated Managers Group, Inc.
Consolidated Balance Sheets
(in thousands)

			December 31, 2002			September 30, 2003
Assets
Current assets:
	Cash and cash equivalents		$	27,708		$	231,093
	Investment advisory fees receivable			50,798			56,215
	Other current assets			11,009			16,902
		Total current assets		89,515			304,210
Fixed assets, net				19,228			37,755
Acquired client relationships, net				374,011			363,885
Goodwill, net				739,053			745,614
Other assets				21,187			23,938
		Total assets	$	1,242,994		$	1,475,402
Liabilities and Stockholders' Equity
Current liabilities:
	Accounts payable and accrued liabilities		$	81,404		$	80,258
	Notes payable to related party			12,348			11,930
		Total current liabilities		93,752			92,188
Senior convertible debt				229,023			423,186
Mandatory convertible securities				230,000			230,000
Deferred income taxes				61,658			83,818
Other long-term liabilities				26,202			18,896
		Total liabilities		640,635			848,088
Minority interest (D)				30,498			33,111
Stockholders' equity:
	Common stock			235			235
	Additional paid-in capital			405,769			407,829
	Accumulated other comprehensive income (loss)			(244	)		714
	Retained earnings			246,444			289,659
				652,204			698,437
Less treasury stock, at cost				(80,343	)		(104,234	)
		Total stockholders' equity		571,861			594,203
		Total liabilities and stockholders' equity	$	1,242,994		$	1,475,402

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9

Affiliated Managers Group, Inc.
Consolidated Statements of Cash Flow
(in thousands)

			Three Months Ended September 30,						Nine Months Ended September 30,
			2002			2003			2002			2003
Cash flow from operating activities:
	Net Income		$	12,819		$	16,395		$	42,680		$	43,215
Adjustments to reconcile Net Income to net cash flow from operating activities:
	Amortization of intangible assets			3,825			4,065			10,521			12,112
	Amortization of debt issuance costs			316			958			2,958			2,414
	Depreciation and other amortization			1,525			1,560			4,327			4,684
	Deferred income tax provision			5,997			8,021			17,033			22,160
	Accretion of interest			290			154			857			559
	Tax benefit from exercise of stock options			1,446			1,506			1,446			2,420
	Other adjustments			62			—			(524	)		(555	)
Changes in assets and liabilities:
	Decrease (increase) in investment advisory fees receivable			14,328			(4,825	)		10,327			(5,417	)
	Increase in other current assets			(1,900	)		(2,360	)		(2,284	)		(3,065	)
	Decrease (increase) in non-current other receivables			(889	)		3,364			(912	)		2,664
	Increase (decrease) in accounts payable, accrued expenses and other liabilities			5,295			13,996			14,057			(770	)
	Increase (decrease) in minority interest			(2,766	)		6,514			(8,585	)		2,613
		Cash flow from operating activities		40,348			49,348			91,901			83,034
Cash flow used in investing activities:
	Purchase of fixed assets			(1,183	)		(20,352	)		(5,050	)		(23,211	)
	Cost of investments, net of cash acquired			(119,025	)		(1,750	)		(134,822	)		(7,868	)
	Investment in marketable securities			—			—			—			(1,852	)
	Increase in other assets			—			—			(213	)		(12	)
	Repayment of loans			1,566			—			1,566			—
		Cash flow used in investing activities		(118,642	)		(22,102	)		(138,519	)		(32,943	)
Cash flow from financing activities:
	Borrowings of senior bank debt			130,000			—			290,000			85,000
	Repayments of senior bank debt			(80,000	)		—			(240,000	)		(85,000	)
	Issuances of debt securities			—			—			30,000			300,000
	Issuances of equity securities			860			4,199			3,453			8,972
	Repayments of notes payable			—			(506	)		—			(8,574	)
	Repurchases of stock			(19,731	)		—			(28,291	)		(33,688	)
	Repurchases of debt securities			—			—			—			(105,841	)
	Debt issuance costs			(2,892	)		(358	)		(4,158	)		(7,819	)
		Cash flow from financing activities		28,237			3,335			51,004			153,050
Effect of foreign exchange rate changes on cash flow				35			—			79			244
Net increase (decrease) in cash and cash equivalents				(50,022	)		30,581			4,465			203,385
Cash and cash equivalents at beginning of period				127,914			200,512			73,427			27,708
Cash and cash equivalents at end of period			$	77,892		$	231,093		$	77,892		$	231,093
Supplemental disclosure of non-cash financing activities:
	Notes issued for Affiliate equity purchases		$	—		$	—		$	12,593		$	938
	Notes received for Affiliate equity sales			—			260			1,800			520
	Capital lease obligations for fixed assets			—			—			—			320
	Common Stock issued in Affiliate equity purchase			—			—			2,113			—
	Common Stock issued in repayment of note			—			—			—			465
	Common Stock received in repayment of loan			—			—			2,263			—

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10

Affiliated Managers Group, Inc.
Notes

(A)

Cash Net Income is defined as Net Income plus amortization and deferred taxes related to intangible assets plus Affiliate depreciation. This supplemental non-GAAP performance measure is provided in addition to, but not as a substitute for, Net Income. The Company considers Cash Net Income an important measure of its financial performance, as management believes it best represents operating performance before non-cash expenses relating to the acquisition of interests in its affiliated investment management firms. Since acquired assets do not generally depreciate or require replacement, and since they generate deferred tax expenses that are unlikely to reverse, the Company adds back these non-cash expenses. Cash Net Income is used by the Company's management and Board of Directors as a principal performance benchmark.

The Company adds back amortization attributable to acquired client relationships because this expense does not correspond to the changes in value of these assets, which do not diminish predictably over time. The Company adds back the portion of deferred taxes generally attributable to intangible assets (including goodwill) that it no longer amortizes but which continues to generate tax deductions. These deferred tax expense accruals would be used in the event of a future sale of an Affiliate or an impairment charge, which the Company considers unlikely. The Company adds back the portion of consolidated depreciation expense incurred by Affiliates because under its Affiliate operating agreements, the Company is generally not required to replenish these depreciating assets.

In connection with its February 2003 issuance of convertible securities, the Company modified its definition to clarify that deferred taxes relating to these convertible securities and certain depreciation are not added back for the calculation of Cash Net Income. In prior periods, Cash Net Income was defined as "Net Income plus depreciation, amortization and deferred taxes." If the Company had used its modified definition of Cash Net Income in 2002, Cash Net Income would have been $23,768 for the three months ended September 30, 2002 and $73,309 for the nine months ended September 30, 2002.

(B)

EBITDA is defined as earnings before interest expense, income taxes, depreciation and amortization. This supplemental non-GAAP liquidity measure is provided in addition to, but not as a substitute for, cash flow from operations. As a measure of liquidity, the Company believes EBITDA is useful as an indicator of its ability to service debt, make new investments and meet working capital requirements. EBITDA, as calculated by the Company, may not be consistent with computations of EBITDA by other companies. In reporting EBITDA by segment, Affiliate expenses are allocated to a particular segment on a pro rata basis with respect to the revenue generated by that Affiliate in such segment.

(C)

Cash earnings per share represents Cash Net Income divided by average shares outstanding. The Company's current presentation of Cash EPS represents a change of the measure as presented in prior years, as discussed in footnote A. If the Company had used its modified definition of Cash EPS in 2002, Cash EPS would have been $1.07 for the three months ended September 30, 2002 and $3.23 for the nine months ended September 30, 2002.

(D)

Minority interest on the Company's income statement represents the profits allocated to Affiliate management owners for that period. Minority interest on the Company's balance sheet represents the undistributed profits and capital owned by Affiliate management, who retain a conditional right to sell their interests to the Company.

(E)

For the three and nine months ended September 30, 2002, this figure represents the Company's total deferred taxes.

(F)

For the three and nine months ended September 30, 2002, this figure represents the Company's consolidated depreciation.

11

QuickLinks

Exhibit 99.1