EXHIBIT 5.1


                              SULLIVAN & WORCESTER LLP
                               ONE POST OFFICE SQUARE
                             BOSTON, MASSACHUSETTS 02109
                                   (617) 338-2800
                                FAX NO. 617-338-2880
     IN WASHINGTON, D.C.                                   IN NEW YORK CITY
1025 CONNECTICUT AVENUE, N.W.                              767 THIRD AVENUE
   WASHINGTON, D.C. 20036                              NEW YORK, NEW YORK 10017
       (202) 775-8190                                       (212) 486-8200
    FAX NO. 202-293-2275                                 FAX NO. 212-758-2151





                                            July 12, 2000


American Tower Corporation
116 Huntington Avenue
Boston, MA  02116

         Re:      Registration of Stock Option Plan Amendment on Form S-8

Ladies and Gentlemen:

         In connection with the  registration  under the Securities Act of 1933,
as amended (the "Act"), by American Tower  Corporation,  a Delaware  corporation
(the "Company"), of 9,000,000 shares of its Class A Common Stock, par value $.01
per share ("Common  Stock"),  that are to be offered under and are issuable upon
the exercise of options granted and to be granted  pursuant to the provisions of
the American Tower Systems Corporation 1997 Stock Option Plan (the "Stock Option
Plan") (all such shares shall be referred to herein as the "Registered Shares"),
the following  opinion is furnished to you to be filed with the  Securities  and
Exchange   Commission  (the  "Commission")  as  Exhibit  5.1  to  the  Company's
registration statement on Form S-8 (the "Registration Statement") under the Act.

         We have  acted  as  counsel  to the  Company  in  connection  with  the
Registration Statement,  and we have examined originals or copies,  certified or
otherwise  identified to our satisfaction,  of the Registration  Statement,  the
Restated  Articles of  Incorporation  of the  Company,  as  presently in effect,
corporate records of the Company, and such other documents as we have considered
necessary in order to furnish the opinion hereinafter set forth.

         We  express no  opinion  herein as to any laws other than the  Delaware
General Corporation Law and the federal law of the United States, and we express
no opinion as to state securities or blue sky laws.

         Based on and subject to the foregoing, we are of the opinion that, when
issued in  accordance  with the terms of the Stock  Option  Plan and the options
granted  thereunder,  the  Registered  Shares will be duly  authorized,  validly
issued, fully paid and nonassessable by the Company.

         We hereby  consent to the  filing of this  opinion as an exhibit to the
Registration  Statement  and to the  reference  to our  firm  in the  Prospectus
forming a part of the Registration Statement.







American Tower Corporation
July 12, 2000
Page 2

         In giving such consent, we do not thereby admit that we come within the
category of persons whose consent is required  under Section 7 of the Act or the
rules and regulations of the Commission promulgated thereunder.

                                                Very truly yours,


                                                /s/ SULLIVAN & WORCESTER LLP