<PAGE>
 
                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549
                        ------------------------------

                                  FORM 10-K/A

[X]      ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES
         EXCHANGE ACT OF 1934

[_]      TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES
         EXCHANGE ACT OF 1934 FOR THE TRANSITION PERIOD FROM ________ TO
         _________

FOR THE YEAR ENDED DECEMBER 31, 1998            COMMISSION FILE NUMBER 000-21091
                              ___________________

                         ADVANCED RADIO TELECOM CORP.
            (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

                   DELAWARE                                     52-1869023
         (STATE OR OTHER JURISDICTION                        (I.R.S. EMPLOYER
       OF INCORPORATION OR ORGANIZATION)                    IDENTIFICATION NO.)

                500 108TH AVENUE, NE
                     SUITE 2600
                BELLEVUE, WASHINGTON                                98004
      (ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)                   (ZIP CODE)
      ----------------------------------------

                                (425) 688-8700
             (REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE)
                                 _____________

          Securities registered pursuant to Section 12(b) of the Act:

                                                       NAME OF EACH EXCHANGE
             TITLE OF EACH CLASS                        ON WHICH REGISTERED
             -------------------                       ---------------------
                    None                                       None

          Securities registered pursuant to Section 12(g) of the Act:

             TITLE OF EACH CLASS:                COMMON STOCK ($.001 PAR VALUE)
             --------------------                                 

     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 the definitive proxy or information
statements incorporated by reference in Part III of this Form 10-K or any
amendment to this Form 10-K [_].

     The aggregate market value of the registrant's voting stock held by
non-affiliates was approximately $271 million on April 12, 1999, based on the
closing sales price of the registrant's common stock as reported on the Nasdaq
National Market as of such date.

     The number of shares outstanding of each of the registrant's classes of
common stock as of April 12, 1999 was as follows:

     Common Stock, $.001 par value:  27,142,409
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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 ON THIS 19TH DAY OF
APRIL 1999.

                                       Advanced Radio Telecom Corp.

                                       /s/ Robert S. McCambridge

                                       By: _________________________________
                                           EXECUTIVE VICE PRESIDENT AND
                                           CHIEF FINANCIAL OFFICER

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