An early phase II study of CPT-11 was carried out in patients with primary lung cancer in 15 institutions throughout Japan. The efficacy and safety of CPT-11 were studied at 200 mg/m2 based on the results of the previous phase I study. Thirty-eight of 52 enrolled patients were eligible. CPT-11 proved to be effective for primary lung cancer. The response rates were 20.0% (7/35) for non-small cell lung carcinoma and 33.3% (1/3 for small cell lung carcinoma. Hematological toxicities included leukopenia (less than or equal to 3,000) in 44.7% of the patients. Other major toxicities were nausea/vomiting (greater than or equal to grade 2) in 50.0% and diarrhea (greater than or equal to grade 2) in 47.4%.