UFT was administered orally at a dosage of 200 mg/day, 2 times a day, to patients over 80 years of age with oral cancer. The concentration of 5-FU in the serum and tumor tissue, as well as the side effects, were investigated. The results were as follows: 1. The concentration of 5-FU in the serum peaked (0.017 to 0.066 microgram/ml) 1 or 2 hours after UFT administration. The concentration 8 hours after administration was relatively high (0.016 to 0.041 microgram/ml). 2. The 5-FU concentrations in the tumor tissues in 3 out of 5 cases were greater than 0.05 microgram/g, which is considered to be the effective level. The concentration tended to be higher with increased duration of administration. 3. A minor side effect, bone marrow dysfunction, was observed. No effect on the function of the liver or digestive system was observed.