The safety of prolonged administration of UFT in which tegafur and uracil were mixed in a ratio of 1:4 in molar fraction was studied in 44 cases of bladder cancer and 10 cases of renal cell carcinoma. Daily doses of UFT were 300-600 mg, and average total doses administered were 102.0 g for bladder cancer and 116.6 g for renal cell carcinoma cases. Incidence of adverse effects were 25.0% in bladder cancer and 18.5% in renal cell carcinoma cases. Anorexia, nausea, vomiting and decrease in WBC were observed, but rates of having discontinued the administration of UFT were very low, being 9.1% in bladder cancer and 10.0% in renal cell carcinoma. Thus, UFT was considered to be tolerable during prolonged use in bladder cancer and renal cell carcinoma and also a drug in which more usefulness is expected in multidisciplinary treatments in future.