The objective of this phase I/II study was to determine the recommended dose (RD) of S-1 and carboplatin (CBDCA), and to evaluate the efficacy and safety of this combination in the treatment of patients with advanced non-small cell lung cancer (NSCLC). Chemotherapy-naïve patients were treated with S-1 given orally on days 1-14, and CBDCA infused intravenously on day 1, repeated every 3 weeks. RD was AUC5 of CBDCA and 80 mg/m(2) of S-1. Nineteen patients were treated at the RD. The overall response was 30.8% (95% confidence interval: 17.1-58.3%). The response rate in the RD was 36.8% (95% CI: 16.3-61.6%). The median overall survival time was 11.1 months (95% CI: 8.1-15.3 months) and the median progression-free survival time was 5.0 months (95% CI: 3.6-6.0 months). Major grades 3-4 toxicities were thrombocytopaenia (47%), anaemia (26%) and infection (16%). This is the first report to show promising activity of this combination in phase II, including survival data and manageable toxicity, especially in outpatients receiving treatment for advanced NSCLC.