As filed with the Securities and Exchange Commission on
                                  April 19, 2001 Registration No. 333-44276
===========================================================================

                     SECURITIES AND EXCHANGE COMMISSION
                           Washington, D.C. 20549

                       POST-EFFECTIVE AMENDMENT NO. 1
                                     TO
                                  FORM S-3
                           REGISTRATION STATEMENT
                                   UNDER
                         THE SECURITIES ACT OF 1933

                                 CMGI, INC.
           (Exact name of Registrant as specified in its charter)


            Delaware                                      04-2921333
(State or other jurisdiction of                        (I.R.S. Employer
 incorporation or organization)                     Identification Number)

                           100 Brickstone Square
                        Andover, Massachusetts 01810
                               (978) 684-3600
     (Address, including zip code, and telephone number, including area
            code, of Registrant's principal executive offices)

                             David S. Wetherell
        President, Chairman of the Board and Chief Executive Officer
                                 CMGI, Inc.
                           100 Brickstone Square
                        Andover, Massachusetts 01810
                               (978) 684-3600
         (Name, address, including zip code, and telephone number,
                including area code, of agent for service)



                                 Copies to:
                                         
       William Williams II                              David T. Brewster
Vice President and General Counsel          Skadden, Arps, Slate, Meagher & Flom LLP
            CMGI, Inc.                                  One Beacon Street
      100 Brickstone Square                        Boston, Massachusetts 02108
   Andover, Massachusetts 01810                          (617) 573-4825
          (978) 684-3600



         Approximate  date of  commencement of proposed sale to the public:
From time to time after this registration statement becomes effective.

         If the only  securities  being  registered  on this Form are being
offered pursuant to dividend or interest  reinvestment  plans, please check
the following box. [_]

         If any of the securities  being  registered on this Form are to be
offered on a delayed or  continuous  basis  pursuant  to Rule 415 under the
Securities Act of 1933,  other than  securities  offered only in connection
with dividend or interest reinvestment plans, check the following box. [_]

         If this Form is filed to  register  additional  securities  for an
offering  pursuant to Rule 462(b) under the Securities Act of 1933,  please
check the following box and list the Securities Act registration  statement
number  of the  earlier  effective  registration  statement  for  the  same
offering. [_]

         If this Form is a post-effective  amendment filed pursuant to Rule
462(c) under the Securities  Act of 1933,  check the following box and list
the Securities Act registration  statement number of the earlier  effective
registration statement for the same offering. [_]

         If delivery of the  prospectus  is expected to be made pursuant to
Rule 434, please check the following box. [_]


                              EXPLANATORY NOTE

         CMGI, Inc. (the  "Registrant") has filed a registration  statement
on Form S-3  (Registration No.  333-44276) (the  "Registration  Statement")
which originally  registered  10,810,911  shares of common stock, par value
$0.01 per share,  of the Registrant for resale by the selling  stockholders
named therein. The offering contemplated by the Registration  Statement has
terminated.  Pursuant  to the  undertaking  contained  in the  Registration
Statement,  the Registrant is filing this Post-Effective Amendment No. 1 to
deregister such number of shares originally  registered by the Registration
Statement as remained unsold as of the termination of the offering.

                                 SIGNATURES

         Pursuant to Rule 478 promulgated under the Securities Act of 1933,
as amended,  the Registrant has duly caused this  Post-Effective  Amendment
No. 1 to the  Registration  Statement  to be  signed  on its  behalf by the
undersigned,   thereunto   duly   authorized,   in  the  Town  of  Andover,
Commonwealth of Massachusetts, on the 19th day of April, 2001.


                                CMGI, INC.

                                By:  /s/ William Williams II
                                   -----------------------------------------
                                     William Williams II
                                     Vice President and General Counsel