<PAGE>
Filed Pursuant to Rule 424(b)(3)
Registration No.: 333-86257
Supplement, Dated April 20, 2000, to Prospectus, Dated September 17, 1999, of
- -----------------------------------------------------------------------------
UnitedGlobalCom, Inc.
- ---------------------
Additional Selling Securityholders. The following selling securityholder is
added to the list of selling securityholders in the prospectus:
<TABLE>
<CAPTION>
Number of Depositary Shares
---------------------------
Name of Selling Securityholder Owned Offered
- -------------------------------- ------------- ------------
<S> <C> <C>
Salomon Smith Barney Inc. 149,600 149,600
</TABLE>
Salomon Smith Barney Inc. was an initial purchaser in the June 1999 private
placement offering through which the selling securityholders acquired their
shares, resulting in material relationships between us and Salomon Smith Barney
Inc.
