We studied whether total gastrectomy for gastric cancer would affect the pharmacokinetics of 5-fluorouracil (5-FU) and its degradation products, such as dihydrouracil (FUH2) and α-fluoro-β-alanine (FBAL), after oral administration of the fluorouracil derivative S-1, composed of tegafur, 5-chloro-2,4-dihydroxypyridine (CDHP; a dihydropyrimidine dehydrogenase inhibitor) and potassium oxonate. Blood and urine samples were obtained, both preoperatively and at least 2 weeks postoperatively, from six patients with advanced gastric cancers who were undergoing total gastrectomy. Plasma levels of tegafur, 5-FU, CDHP, potassium oxonate, FUH2 and FBAL were measured prior to and at 1, 2, 4, 6 and 10 h after oral administration of 40 mg/m2 S-1. The total amounts of 5-FU, FUH2 and FBAL excreted into urine during the 24-h period after S-1 administration were also measured. Total gastrectomy significantly increased the maximum concentration and the area under the curve until 10 h after administration (AUC1–10h) of plasma 5-FU. The plasma AUC1–10h of CDHP was significantly higher than the preoperative value. In terms of clinical efficacy, the higher AUC1–10h of 5-FU after total gastrectomy may be beneficial to S-1 administered as adjuvant chemotherapy, and might be caused by the higher postoperative AUC1–10h of CDHP relative to preoperative values. However, the dose of S-1 for patients who have undergone total gastrectomy might be diminished to avoid severe adverse events and to continue the treatment for a long period. (Cancer Sci 2007; 98: 1604–1608)