<PAGE>

                                 UNITED STATES
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                 ------------

                               FORM 10-K/A

                                 ------------

    (Mark One)
 Annual Report Pursuant to Section 13 or 15(d) of the Securities Exchange Act
                            of 1934 [Fee Required]
      [X]

                 For the Fiscal Year Ended September 30, 1999

                                      OR

      [_]
 Transition Report Pursuant to Section 13 or 15(d) of the Securities Exchange
                         Act of 1934 [No Fee Required]

                   For the transition period from     to
<TABLE>
<CAPTION>
   Commission        Registrant, State of Incorporation            IRS Employer
   File Number          Address and Telephone Number            Identification No.
   -----------       ----------------------------------         ------------------
   <S>               <C>                                        <C>
   33-27835-01         AmeriSource Health Corporation               23-2546940
                       (a Delaware Corporation)
                       300 Chester Field Parkway
                       Malvern, PA 19355-
                       (610) 296-4480
</TABLE>

Securities Registered Pursuant to Section 12(b) of the Act:
                                             AmeriSource Health Corporation:
                                             None

Securities Registered Pursuant to Section 12(g) of the Act:
                                             AmeriSource Health Corporation:
                                             Common Stock, $.01 par value per
                                             share


Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to
such filing requirements for the past 90 days. Yes [X]  No [_]

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405
of Regulation S-K is not contained herein, and will not be contained, to the
best of the registrant's knowledge, in definitive proxy or information
statements incorporated by reference in Part III of this Form 10-K or any
amendment to this Form 10-K. [_]

Non-affiliates of AmeriSource Health Corporation, as of December 1, 1999, held
49,092,588 shares of voting stock. The registrant's voting stock is traded on
the New York Stock Exchange under the trading symbol "AAS." The aggregate
market value of the registrant's voting stock held by non-affiliates of the
registrant (based upon the closing price of such stock on the New York Stock
Exchange on December 1, 1999 and the assumption for this computation only that
399 Venture Partners, Inc. and all directors and executive officers of the
registrant are affiliates) was $610,589,063.25.

The number of shares of common stock of AmeriSource Health Corporation
outstanding as of December 1, 1999 was: Class A--51,178,274; Class B--8,446;
Class C--165,346.

                      Documents Incorporated by Reference

Portions of the following document are incorporated by reference in the Part
of this report indicated below:

Part III--Registrant's Proxy Statement for the 2000 Annual Meeting of
 Stockholders.
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                                  SIGNATURES

  Pursuant to the requirements of Section 13 or 15(d) 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.

                                          AmeriSource Health Corporation

                                                  /s/ George L. James III
                                          By: _________________________________
Date: December 29, 1999                         (George L. James III) Vice
                                               President and Chief Financial
                                                          Officer

  Pursuant to the requirements of the Securities Exchange Act of 1934, this
report has been signed below on December 29, 1999 by the following persons on
behalf of the Registrant and in the capacities indicated.

              Signature                               Title

          /s/ R. David Yost                   President and Chief
-------------------------------------          Executive Officer
           (R. David Yost)                     and Director
                                               (Principal
                                               Executive Officer)

       /s/ George L. James III                Vice President and
-------------------------------------          Chief Financial
        (George L. James III)                  Officer (Principal
                                               Financial Officer)

      /s/ Michael D. DiCandilo                Vice President,
-------------------------------------          Controller
       (Michael D. DiCandilo)                  (Principal
                                               Accounting Officer)

                                              Director
     /s/ Bruce C. Bruckmann
-------------------------------------
        (Bruce C. Bruckmann)

       /s/ Michael A. Delaney                 Director
-------------------------------------
        (Michael A. Delaney)

        /s/ Richard C. Gozon                  Director
-------------------------------------
         (Richard C. Gozon)

       /s/ Lawrence C. Karlson                Director and
-------------------------------------          Chairman
        (Lawrence C. Karlson)

        /s/ George H. Strong                  Director
-------------------------------------
         (George H. Strong)

      /s/ Edward E. Hagenlocker               Director
-------------------------------------
       (Edward E. Hagenlocker)

                                              Director
-------------------------------------
         (Barton J. Winokur)
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                                EXPLANATORY NOTE

The purpose of this amendment filing is solely to bring the EDGAR database
current. Exhibit 23 was inadvertently not included in the Form 10-K filing
submitted on December 28, 1999 (Registration No. 000-20485).