A combination Phase I study of gemcitabine and irinotecan in patients with previously untreated advanced non-small-cell lung cancer was conducted. Patients received gemcitabine and irinotecan on Days 1 and 8 every 3 weeks. A total of 11 patients were enrolled. Three of six patients who received the starting dose (gemcitabine, 800 mg/m(2); irinotecan, 80 mg/m(2)) experienced dose-limiting toxicities (Grade 4 neutropenia, Grade 3 elevation of transaminase and Grade 5 interstitial pneumonia). At the reduced dose level (gemcitabine, 800 mg/m(2); irinotecan, 60 mg/m(2)), all two assessable patients could not meet the administration criteria of Day 8 (one, Grade 2 elevation of transaminase; the other, Grade 1 diarrhea). No objective response was observed in eight evaluable patients. We could not determine the recommended dose of this combination because of intolerable toxicities and no efficacy. Therefore, it is difficult to forward this combination chemotherapy toward further studies.