The efficacy and safety of irinotecan (CPT-11) combined with cisplatin (CDDP) were assessed in 24 previously untreated patients with primary non-small cell lung cancer. CPT-11 (60 mg/m2) and CDDP (30 mg/m2) were administered in combination at weekly intervals, on days 1.8, and 15 of the treatment course. During treatment, the patients were evaluated for adverse drug reactions and response. The response rate was 58.3% for all patients and 60.9% for the patients who completed the full treatment course. The median survival time was 13.0 months. The major adverse reactions were myelosuppression and gastro-intestinal disorders, but no treatment-related deaths were observed. Myelosuppressions included grade 3 or 4 leukopenia (25.0%) and anemia (33.3%). Grade 3 and higher gastro-intestinal reactions included nausea and vomiting (8.3%), diarrhea (12.5%), and anorexia (16.7%). These results suggest that combined weekly CPT-11 and CDDP therapy is capable of achieving a favorable tumor response with less toxicity, and thus worth consideration as a treatment option. Given that only 33.3% of the patients finished the full treatment course, further study should be devoted to the subject of CDDP and/or CPT-11 dosages.