A clinical study of weekly administration of CPT-11, a semi-synthetic derivative of Camptothecin, was performed in patients with advanced lung cancer to determine the optimal dose for a weekly single dose schedule. Sixteen out of 19 patients enrolled were evaluable. The starting dose was 50 mg/m2 and gradually escalated to 100, 125 and 150 mg/m2. CPT-11 was given by intravenous infusion for 90 minutes every week. The maximum tolerated dose for this schedule was estimated to be 125 mg/m2. The dose-limiting toxicity was leukopenia with median nadir of 2,900/mm3, median day to nadir of 21, and median day to recovery of 7. Other major toxicity was gastrointestinal upset, but was mild and tolerable. Objective tumor responses were observed in four patients, three with non-small cell carcinoma and one with double cancer (small and non-small cell carcinoma). The responding patients were treated at a dosage of 100 mg/m2 or more. The recommended dose for a phase II study is considered to be 100 mg/m2 iv infusion for a weekly single-dose schedule.