Novel N-phthalidyl-5-fluorouracil derivatives represen- led by the following formula:
wherein one of R, and R2 represents a phthalidyl group of
(wherein R3 represents a hydrogen atom, a halogen atom, an alkoxy group or an alkyl group) and the other represents a hydrogen atom, an acyl group or a cyclic ether group of
(n: 1 or 2), or R, and R2 represent the same phthalidyl group of
(wherein R3 is the same as defined above). They have antineoplastic effects with less toxicity, thus used as carcinostatic agents.