The 5-fluorouracil (5-FU)-concentration in resected tumor tissues was compared with that in the adjacent normal tissues or that in serum. Twenty-three patients with cancer of the digestive organs [(carcinoma of the stomach (12), colon (6), gallbladder (1), liver (1), ampulla of Vater (1), bile duct (1) and pancreas (1)] were administered orally 600-1, 200 mg/day of 5'-deoxy-5-fluorouridine (5'-DFUR) for 3 to 5 consecutive preoperative days. The average 5-FU-concentration in tumor tissues of 14 patients with 16 specimens measured 5 hours after final administration of 5'-DFUR was 62 ng/g. This level was significantly higher than that in adjacent normal tissues (21 ng/g, p less than 0.01). With 10 out of these 14 patients, the 5-FU-concentration in the serum was also measured. Among these 10 patients, the concentration of 5-FU in tumors (54 ng/g) was significantly higher than in adjacent normal tissues (19 ng/g) and in serum (12 ng/ml). At 6 to 9 hours after final administration, the mean concentration of 5-FU was 33 ng/g (n = 4) in tumor tissues, 30 ng/g (n = 5) in adjacent normal tissues and 24 ng/ml in serum. There was no significant difference among these mean values. For 4 patients whose specimens were taken at 17 to 18 hours after the last 5-FU administration, the mean concentration of 5-FU was 29 ng/g (n = 4) in tumor tissues, 14 ng/g (n = 3) in adjacent normal tissues and 4 ng/ml (n = 2) in serum.