Gefitinib, an epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI), is an effective treatment for advanced non-small cell lung cancer (NSCLC) in patients with activating EGFR mutations. However, there have been little evidence-based studies of gefitinib in combination with platinum-doublet therapy in these patients. We performed a phase II trial to determine the efficacy and safety of triplet chemotherapy with gefitinib, carboplatin, and S-1 as a first-line treatment. This was a multicentre, single-arm, phase II trial of carboplatin, S-1, and gefitinib in advanced NSCLC patients with activating EGFR mutations. Patients received four courses of these drugs in 3-4 week cycles. In each cycle, carboplatin (area under curve = 5) was administered on day 1, S-1 (80 mg/m(2)) on days 1-14, and gefitinib (250 mg) every day. Subsequently, the same regimen without carboplatin was administered until disease progression or unacceptable toxicity occurred. The 1-year progression-free survival (PFS) was the primary endpoint, while response rate (RR), PFS, overall survival (OS), and safety were secondary endpoints. Thirty-five patients were enrolled into this study. The 1-year PFS was 74.3% and the overall RR was 85.7%. The median PFS for all patients was 17.6 months (95% confidence interval 15.5-∞), but the median OS was not reached, because 28 patients were still alive after a median follow-up time of 21.4 months. Haematological adverse events (grade 3 or higher) included neutropaenia (17.1%), thrombocytopenia (14.3%), and anaemia (5.7%), while non-haematological adverse events (grade 3 or higher) included elevated aminotransferase (20.0%), diarrhoea (14.3%), and febrile neutropaenia (2.9%). No interstitial lung disease or treatment-related deaths occurred. Combination chemotherapy with carboplatin, S-1, and gefitinib is efficacious and well tolerated as a first-line treatment in advanced NSCLC patients with activating EGFR mutations.