A dose-escalation study of irinotecan (CPT-11) combined with S-1, an oral dihydropyrimidine dehydrogenase inhibitory fluoropyrimidine, was performed to determine the maximum-tolerated dose (MTD), recommended dose (RD), dose-limiting toxicities (DLTs), and objective response rate (RR) in advanced gastric cancer (AGC). S-1 was administered orally at 80 mg m-2 day-1 from day 1 to 14 of a 28-day cycle and CPT-11 was given intravenously on day 1 and 8 at an initial dose of 70 mg m-2 day-1, stepping up to 100 mg m-2. The treatment was repeated every 4 weeks, unless disease progression was observed. In the phase I portion, the MTD of CPT-11 was presumed to be 100 mg m-2, because 66.6% of patients (two of three) developed DLTs. All three patients at the initial RD of CPT-11 (90 mg m-2) experienced grade 4 haematological or grade 3 nonhaematological toxicities at second course, followed by the dose reduction of CPT-11 from the third course. Considering safety and the ability to continue treatment, the final RD was determined to be 80 mg m-2. In the phase II portion, 42 patients including seven patients in the final RD phase I portion were evaluated. The median treatment course was five (range: 1-13). The incidences of severe (grade 3-4) haematological and nonhaematological toxicities were 19 and 10%, respectively, but all were manageable. The RR was 62% (26 of 42, 95% confidence interval: 47.2-76.6%), and the median survival time was 444 days. Our phase I/II trial showed S-1 combined with CPT-11 is effective for AGC and is well tolerated, with acceptable toxicity.