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Background: We aimed to determine the maximum-tolerated dose (MTD) of S-1 when given with oxaliplatin, to evaluate S-1 pharmacokinetics, and to determine the efficacy and safety of this regimen as a first-line treatment for advanced gastric cancer (AGC)., Methods: Oxaliplatin was fixed at a dose of 130 mg/m2 on day 1 (D1). S-1 was administered from D1 to D14 of a 3-week cycle, and escalated by 10 mg/m2 per day from 70 mg/m2 per day up to 100 mg/m2 per day. Pharmacokinetic analyses were performed following a single dose of S-1 on D-5 and D1 of the first cycle., Results: In phase I (n=18), MTD was not defined. In phase II (n=47) with the planned maximum dose, partial response was achieved in 26 patients (55.3%) and stable disease in 14 patients (29.8%). The median time to progression was 6.6 months (95% CI 4.0-9.2 months) and the median overall survival was 12.5 months (95% CI 9.2-15.9 months). Frequent grade 3/4 toxicities included thrombocytopenia (39%), neutropenia (28%), anemia (17%), and leukopenia (13%). There was one grade 5 febrile neutropenia during the first cycle., Conclusions: The pharmacokinetics of S-1 was not influenced by oxaliplatin. S-1/Oxaliplatin combination therapy is highly active against AGC and has a favorable toxicity profile.