We studied the pharmacokinetics of 5-FU after S-1 oral administration at the usual dose (80 mg/m2) for adjuvant chemotherapy in 13 advanced gastric cancer patients (Stage II, III), and at a decreased dose (60 mg/m2) for adjuvant or combined chemotherapy in 13 advanced gastric cancer patients. Pharmacokinetic parameters of 5-FU in the serum were as follows: Cmax, 159 .9 2+/-45.2 ng/mL, Tmax, 2.17+/-0.58 h;T1/2, 3.13+/-2.88 h; and AUC(0-8), 768.0+/-260.8 ng h/mL in the patients with the usual dose, and Cmax, 117.3+/-55.1 ng/mL; Tmax, 2.62+/-0.9 6 h; T1/2, 3.09+/-1.9 5 h and AUC(0-8), 565.9+/-216.8 ng h/mL in the patients with the decreased dose. No difference in AUC was observed between operative methods. Adverse events of more than grade 3 were recognized in 7 patients, and AUC of 6 patients were more than 800 ng h/mL. The plasma concentration of 5-FU was quite different between patients. The difference of Cmax and AUC was 3-4 times. It was concluded that we must pay attention to individual differences in the plasma concentration of 5-FU in postoperative gastric cancer patients when S-1 would be administered.