TT-62 is a new derivative of FdUMP, which is the active metabolite of 5-FU. A phase I clinical study of TT-62 was conducted by a cooperative study. The same patients received single and 2-week oral administration of TT-62. Starting from 60 mg/m2 (1n), the dose was escalated to 420 mg/m2 (7n). In the single administration, the maximum tolerated dose (MTD) could not be determined. In the 2-week administration, MTD was 420 mg/m2, and the dose limiting factor was gastro-intestinal disturbances such as anorexia, nausea, vomiting and diarrhea. Increases in GOT.GPT and a decrease in hemoglobin content were observed. After administration was stopped all side effects disappeared. TT-62 was detected mainly in the plasma, while trace amounts of 5-FU and FUdR were also detected. TT-62 was excreted mostly in the urine, as alpha-fluoro-beta-alanine (FBAL). The cumulative urinary excretion of FBAL was about 80% of the total dose, and the oral absorption of TT-62 was thus thought to be good.