Fifty-three patients with advanced colorectal cancer were given single oral doses of 5'-DFUR, 400 mg/body, preoperatively to assess the pharmacokinetics of 5'-DFUR in the patients with colorectal cancer and to investigate schedules for 5'-DFUR treatment. The time courses of PyNPase activity and 5-FU concentration in the resected tissue specimens were determined. This study indicated that: 1) High 5-FU concentration and PyNPase activity were noted in tumor tissue and lymph nodes. 2) Effective 5-FU concentration in tumor tissue was maintained even 24 hours after treatment. 3) Effective lymph node concentration of 5-FU was maintained even 8 hours after treatment. 4) PyNPase activity in tumor tissue was significantly higher than that in the normal intestinal mucosa (p < 0.05). These results suggest that 400 mg of 5'-DFUR at 8-hour intervals provides effective tissue concentrations of 5-FU for the treatment of patients with advanced colorectal cancer.