Two phase I trials of irinotecan (CPT-11) in combination with cisplatin were conducted. In both cases, the dose-limiting toxicities were leukopenia and/or diarrhea. During these trials the pharmacokinetics of CPT-11 and its active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), were investigated to evaluate the relationship between pharmacokinetic parameters and diarrhea, since this is an unpredictable and severe toxicity of combination chemotherapy using CPT-11 and cisplatin. Twenty-three previously untreated patients with advanced lung cancer were evaluated in the pharmacokinetic study. Ten patients received CPT-11 at 80 or 90 mg/m2 plus cisplatin at 60 mg/m2. The other 13 patients received CPT-11 at 80 or 90 mg/m2 plus cisplatin at 80 mg/m2 with the granulocyte colony-stimulating factor support (2 micrograms/kg x 16 days). CPT-11 was given as a 90-min intravenous infusion on days 1, 8, and 15. Cisplatin was given on day 1. The pharmacokinetics of CPT-11 and SN-38 were analyzed on day 8 during the first course of treatment. The maximum tolerated dose of CPT-11 was 90 mg/m2 in both phase I trials. The severity of diarrhea was best correlated with the peak plasma concentration of SN-38 among the pharmacokinetic parameters tested. In addition, patients with a plasma SN-38 level > 12.4 ng/ml at 1.75 h after the start of CPT-11 infusion had a higher incidence of Eastern Cooperative Oncology Group grade 3-4 diarrhea than those with a lower SN-38 level (P = 0.0003). Stepwise logistic regression analysis identified the SN-38 concentration as a significant contributor to the development of diarrhea (P = 0.0021). We conclude that there is a clear relationship between the SN-38 concentration and diarrhea during chemotherapy with CPT-11 plus cisplatin.