<Page> UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, DC 20549 ------------------ 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): September 10, 2003 AMERICAN MEDICAL SECURITY GROUP, INC. (Exact name of Registrant as specified in its charter) WISCONSIN 1-13154 39-1431799 (State of Incorporation) (Commission File Number) (I.R.S. Employer Identification No.) 3100 AMS BOULEVARD, GREEN BAY, WISCONSIN 54313 (Address of principal executive offices) (Zip Code) (920) 661-1111 (Registrant's telephone number, including area code) ------------------ <Page> ITEM 5. OTHER EVENTS AND REGULATION FD DISCLOSURE On September 10, 2003, American Medical Security Group, Inc. announced the sale of its preferred provider organization subsidiary, Accountable Health Plans of America, Inc., to IPACQ, Inc. A copy of the press release issued by the Company related to the sale is attached hereto as Exhibit 99 and incorporated herein by reference. ITEM 7. FINANCIAL STATEMENTS AND EXHIBITS (c) EXHIBITS See the Exhibit Index following the Signature page of this report, which is incorporated herein by reference. 2 <Page> SIGNATURE 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 thereunto duly authorized. AMERICAN MEDICAL SECURITY GROUP, INC. Dated: September 11, 2003 /s/ Timothy J. Moore --------------------------------------------- Senior Vice President of Corporate Affairs, General Counsel & Secretary 3 <Page> AMERICAN MEDICAL SECURITY GROUP, INC. (the "Registrant") (Commission File No. 1-13154) EXHIBIT INDEX TO FORM 8-K CURRENT REPORT Date of Report: September 10, 2003 EXHIBIT FILED NUMBER DESCRIPTION HEREWITH - ------ ----------- -------- 99 Press Release dated September 10, 2003, issued by the X Registrant EX-1