We designed a phase I/II trial of S-1 combined with weekly docetaxel to determine the maximum tolerated dose (MTD) and recommended dose (RD) and to evaluate the efficacy and toxicity in metastatic gastric carcinoma (MGC). Patients with measurable disease received S-1 orally b.i.d. on days 1-14 and docetaxel intravenously on days 1 and 8 every 3 weeks. In phase I (n=30), each cohort received escalating doses of S-1 (30-45 mg m(-2) b.i.d.) and docetaxel (25-40 mg m(-2)); MTD was 45 mg m(-2) b.i.d. S-1/35 mg m(-2) docetaxel and RD was 40 mg m(-2) b.i.d. S-1/35 mg m(-2) docetaxel. Dose-limiting toxicities included grade 3 elevated liver enzymes, gastric perforation, grade 3 diarrhoea/fatigue, febrile neutropenia with grade 3 anorexia/fatigue, and neutropenic infection with grade 3 stomatitis/anorexia. In phase II (n=52), the overall response rate was 66.7% (95% confidence interval (CI): 53.8-79.6%) and the median time to progression and overall survival were 6.5 months (95% CI: 4.9-8.1) and 13.7 months (95% CI: 9.9-17.5), respectively. The most common grade 3/4 toxicity was neutropenia (29.4%), and febrile neutropenia/neutropenic infection occurred in 19.6% of patients. Non-haematological toxicities were generally mild. There was one treatment-related death due to pneumonitis. S-1 combined with weekly docetaxel is active in MGC with moderate toxicities.