A Phase II study of CPT-11, a new camptothecin, was performed in patients with primary lung cancer. Patients with previously untreated non-small cell carcinomas (group A), or previously treated non-small cell carcinomas (group B), and with small cell carcinomas (group C), were enrolled in this study. CPT-11 was given at a dose of 100 mg/m2 i.v. infusion once a week for three weeks or more. Out of 153 patients enrolled, 128 (A: 67; B: 26; C: 35) were assessed to be evaluable for response by an extramural review committee. Response rates were 34.3% (23/67) for A, 0% (0/26) for B and 37.1% (13/35) for C. The response rate was 50% for previously untreated patients (4/8), and 33.3% for previously treated patients (9/27) including 2 complete responses in the group C. Major toxicities were leukopenia, nausea/vomiting, diarrhea, anorexia and alopecia. Leukopenia and diarrhea were considered to be dose limiting toxicities, but they were reversible. It was, however, suggested that some patients should be monitored carefully for severe reactions and delay in recovery. The results showed that CPT-11 was highly effective against non-small cell and small cell carcinomas of the lung.