Background: Recently, there have been several studies showing that irinotecan hydrochloride, a topoisomerase I inhibitor, is effective against extensive disease(ED) small cell lung cancer (SCLC). We conducted a phase II trial to evaluate the efficacy and toxicity of irinotecan plus cisplatin as a 1st line therapy for both limited and extensive disease SCLC. Methods: The study was conducted between January 2002 and June 2004. Patients were treated with 60mg/m2 irinotecan on day 1, 8, 15 and 60mg/m2 cisplatin on day 1, every 4 weeks. During concurrent thoracic irradiation for limited disease (LD)-SCLC patients, dose of irinotecan was reduced to 40mg/m 2. Prophylactic cranial irradiation was given to patients with complete remission (CR) after chemotherapy. Results: Median ages of LD- and ED- SCLC were 64 years and performance status (PS) was 0-2. In patients with LD-SCLC, the response rate after concurrent chemoradiotherapy was 85% (CR, 6; Partial response [PR], 11). The median survival was 20 months (95% CIs, 15.6 to 24.4) with 1- and 2-year survival rates of 85% and 35%, respectively. Median progression free survival (PFS) was 12 months (95% CIs, 6.2 to 18.1) with 1- year PFS of 36%. In ED-SCLC, the response rate was 83.4% (CR, 1; PR, 14). The median survival was 14.5 months (95% CIs, 8.8 to 20.1) with 1-year survival rates of 75% Median PFS was 6.3 months (95% CIs, 5.6 to 7.1) with 1- year PFS of 20% The major toxicities (grade 3 or 4) of this regimen included leukopenia, anemia, thrombocytopenia, nausea/vomiting, and diarrhea without life threatening complication. Conclusion: Our data shows that the combination of irinotecan plus cisplatin as a first line therapy is effective and tolerable in the treatment of both LD- and ED- SCLC.