<PAGE>
FILED PURSUANT TO RULE 424(b)(3)
REGISTRATION NO. 333-86257
Supplement, Dated February 10, 2000, to Prospectus, Dated September 17, 1999, of
- --------------------------------------------------------------------------------
UnitedGlobalCom, Inc.
- ---------------------
Additional Selling Securityholders. The following selling securityholders are
added to the list of selling securityholders in the prospectus:
Number of Depositary Shares
---------------------------
Name of Selling Securityholder Owned Offered
- ----------------------------------------------- ------------- ------------
Donaldson, Lufkin & Jenrette Securities Corp. 40,000 40,000
Salomon Smith Barney Inc. 125,100 125,100
Donaldson, Lufkin & Jenrette Securities Corp. and Salomon Smith Barney Inc. were
initial purchasers in the June 1999 private placement offering through which the
selling securityholders acquired their shares, resulting in material
relationships between us and the initial purchasers.
