5-Fluorouracil derivatives represented by the formula ##SPC1## Wherein R is arylcarbonyl, substituted arylcarbonyl, heterocyclic carbonyl, alkylsulfonyl, arylsulfonyl, substituted arylsulfonyl, heterocyclic sulfonyl or alicyclic sulfonyl are effective antimetabolites useful in mammary gland or gastrointestinal cancer therapy.