BACKGROUND: Irinotecan hydrochloride, a topoisomerase I inhibitor, is effective against small-cell lung cancer. In a phase 2 study of irinotecan plus cisplatin in patients with extensive small-cell lung cancer, there was a high response rate and a promising median survival time. METHODS: We conducted a multicenter, randomized, phase 3 study in which we compared irinotecan plus cisplatin with etoposide plus cisplatin in patients with extensive (metastatic) small-cell lung cancer. RESULTS: The planned size of the study population was 230 patients, but enrollment was terminated early because an interim analysis found a statistically significant difference in survival between the patients assigned to receive irinotecan and cisplatin and those assigned to receive etoposide and cisplatin; as a result, only 154 patients were enrolled. The median survival was 12.8 months in the irinotecan-plus-cisplatin group and 9.4 months in the etoposide-plus-cisplatin group (P=0.002 by the unadjusted log-rank test). At two years, the proportion of patients surviving was 19.5 percent in the irinotecan-plus-cisplatin group and 5.2 percent in the etoposide-plus-cisplatin group. Severe or life-threatening myelosuppression was more frequent in the etoposide-plus-cisplatin group than in the irinotecan-plus-cisplatin group, and severe or life-threatening diarrhea was more frequent in the irinotecan-plus-cisplatin group than in the etoposide-plus-cisplatin group. CONCLUSIONS: Irinotecan plus cisplatin is an effective treatment for metastatic small-cell lung cancer.