<PAGE>
FILED PURUSANT TO RULE 424(B)(3)
REGISTRATION NO. 333-86257
Supplement, Dated January 12, 2000, to Prospectus, Dated September 17, 1999, of
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UnitedGlobalCom, Inc.
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Additional Selling Securityholders. The following selling securityholders are
added to the list of selling securityholders in the prospectus:
<TABLE>
<CAPTION>
Number of Depositary Shares
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Name of Selling Securityholder Owned Offered
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<S> <C> <C>
MFS Series Trust I - MFS Convertible Securities Fund.............. 230 230
MFS Series Trust V - MFS Total Return Fund........................ 40,000 40,000
MFS Series Trust VI - MFS Utilities Fund.......................... 135,600 135,600
MFS Series Trust VII - MFS Capital Opportunities Fund............. 212,420 212,420
MFS/Sun Life Series Trust - MFS/Sun Life-Capital Opportunities
Series........................................................... 23,900 23,900
MFS/Sun Life Series Trust - MFS/Sun Life-Utilities Series......... 34,000 34,000
MFS Variable Insurance Trust - MFS VIT-MFS Capital
Opportunities Series............................................. 3,300 3,300
MFS Variable Insurance Trust - MFS VIT-MFS Utilities Series....... 15,100 15,100
Portfolio Partners Inc. Value Fund................................ 18,200 18,200
</TABLE>
