8-K

QuickLinks -- Click here to rapidly navigate through this document

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): January 28, 2004

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 January 28, 2004.

Item 9. Regulation FD Disclosure.

        The following information is furnished pursuant to Item 9, "Regulation FD Disclosure." On January 28, 2004, Registrant issued a press release regarding a stock split of its outstanding shares of Common Stock. A copy of this press release is attached hereto as Exhibit 99.1 and hereby incorporated by reference. By this filing, Registrant is not establishing the practice of filing all press releases in the future and may discontinue such filings at any time.

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: January 28, 2004	/s/  DARRELL W. CRATE DARRELL W. CRATE Executive Vice President, Chief Financial Officer and Treasurer (and also as Principal Financial and Accounting Officer)

QuickLinks

Item 7. Exhibits.
Item 9. Regulation FD Disclosure.

SIGNATURES

EX-99.1

QuickLinks -- Click here to rapidly navigate through this document

Exhibit 99.1

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

AMG Announces Three-for-Two Stock Split

Boston, MA, January 28, 2004—Affiliated Managers Group, Inc. (NYSE: AMG) announced that its Board of Directors has authorized a three-for-two stock split of AMG's outstanding shares of Common Stock. The split will entitle each shareholder of record as of February 24, 2004 to receive three shares for every two shares of Common Stock held on the record date.

The additional shares of Common Stock will be distributed on March 29, 2004. "This stock split reflects our confidence in AMG's prospects for continued strong growth, through both the internal growth of our existing Affiliates, as well as additional investments in high quality mid-sized asset management firms," stated William J. Nutt, Chairman and Chief Executive Officer. "The split will also benefit our shareholders by enhancing the marketability and broadening the distribution of AMG Common Stock."

AMG is an asset management company that acquires and holds majority equity investments in a diverse group of mid-sized investment management firms. AMG's affiliated investment management firms managed approximately $92 billion in assets as of December 31, 2003.

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 ability to complete pending acquisitions, 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.

###

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

QuickLinks

Exhibit 99.1