Cisplatin and irinotecan are reported to act synergistically. The authors have conducted a phase II trial combining cisplatin and irinotecan in patients with refractory lung cancer to evaluate the activity and safety of the regimen. Twenty-one patients, who had not responded to prior platinum-based chemotherapy, were entered into the study. Both cisplatin (30 mg/m2) and irinotecan (60 mg/m2) were administrated on days 1, 8, and 15, and the regimen was repeated every 28 days. There were six partial responses, and the response rate was 29% (95% confidence interval, 11.3%-52.2%). The median survival time of all patients was 32 weeks, and 1-year and 2-year survival rates were 43% and 11%, respectively. Major toxicities were hematologic; leukopenia of grades 3 and 4 developed in 43% patients, anemia developed in 38%, and thrombocytopenia developed in 19%. One notable nonhematologic toxicity was diarrhea; which was grade 3 in 38%. The weekly chemotherapy combining cisplatin and irinotecan was active against lung cancer, which is refractory to platinum-based chemotherapy. However, skips of drug administration or dose reduction were necessary in 76% patients during two courses of planned administration, though the ratio of actual dose to scheduled dose was 81%. Dose modification would be necessary to yield better results by this weekly chemotherapy.