5'-O-(L-valyl)-5-fluorouridine hydrochloride (FO-152) is a derivative of 5-fluorouridine. In preclinical studies, FO-152 has demonstrated more favorable antitumor effect with greater therapeutic index compared with the parent compound, 5-fluorouridine. A phase I study was performed in 38 cases with advanced cancer. Myelosuppression was dose limiting, with leukopenia more pronounced than thrombocytopenia. Other side effects included mild hot feeling of the body during infusion and slight G.I. toxicities. Clinical responses were seen in patients with adenocarcinoma of the stomach, lung, etc. FO-152 has major advantages over 5-fluorouridine in terms of reduced toxicity. The recommended phase II dose for patients with solid tumors is 250 mg/m2 X 1, or 35 mg/m2 X 5 consecutive days with a 4-week rest, provided there is reversal of drug related myelosuppression.