Using the cross-over method, the same patients were administered continuous intravenous injections of 5-FU and HCFU, an oral derivative of 5-FU widely used for breast and colon cancer in Japan. The pharmacokinetics of 5-FU in blood of both drugs were then compared. The AUC of the 5-FU concentration in blood of the HCFU 100 mg group tended to be higher than that in the 5-FU 250 mg group and lower than that in the 5-FU 500 mg group. There was, however, no statistically significant difference between the HCFU and 5-FU 250 mg groups (p = 0.3274), or between the HCFU and 5-FU 500 mg groups (p = 0.1921).