Orally administrable 5-fluorouracil derivatives of the general formula: ##STR1## [wherein R.sup.1 is aryl or substituted aryl; R.sup.2 is acyclic or cyclic alkyl, adamantyl, aryl or substituted aryl; R.sup.3 and R.sup.4 are the same or different, each representing hydrogen atom or lower alkyl] effective against malignant tumors.