We have conducted a Phase I trial to determine the maximum tolerated dose of CPT-11 together with a fixed dose of cisplatin in patients with advanced lung cancer, and the dose-limiting toxicities of this combination. Fourteen previously untreated patients with stage IIIB or IV disease were treated with CPT-11 (90-min intravenous infusion on days 1, 8, and 15) plus cisplatin (60 mg m-2, intravenously on day 1). The starting dose of CPT-11 was 60 mg m-2, and diarrhea was the dose-limiting toxicity at the 90 mg m-2 dose level. All three patients (all four cycles) given 90 mg m-2 of CPT-11 experienced grade 3 diarrhea. Hematologic toxicity was relatively mild. Elimination of CPT-11 was biphasic with a mean (+/- s.d.) beta half-life of 11.36 +/- 7.26 h. The mean terminal half-life of the major metabolite (7-ethyl-10-hydroxycamptothecin; SN-38) was 22.13 +/- 13.28 (s.d.) h, and modest escalation of the CPT-11 dose from 80 mg m-2 to 90 mg m-2 resulted in a statistically significant apparent increase in the plasma concentrations of SN-38. There were one complete response (7%) and five partial responses (36%) among the 14 patients for an overall response rate of 43%. The recommended dose for Phase II studies is 80 mg m-2 of CPT-11 and 60 mg m-2 of cisplatin.