['To determine the response rate, survival, and toxicity of the new anticancer agent, irinotecan (CPT-11), in the treatment of metastatic colorectal cancer.', 'Forty-one chemotherapy-naive patients with measurable metastatic colorectal cancer were treated with a 90-minute infusion of irinotecan 125 mg/m2 administered weekly for 4 weeks every 6 weeks. Pretreatment tumor biopsies to assess topoisomerase-I (Topo-I) activity were obtained from 11 patients. The pharmacokinetics for irinotecan and its active metabolite, SN-38, were determined in 18 patients.', 'Thirteen of 41 patients (32%) had a partial response (PR; 95% confidence interval, 18% to 46%). The median response duration was 8.1 months (range, 4.0 to 16.0) and the median survival time was 12.1 months (range, 2.1 to 21.7) for all 41 patients. Grade 3 or 4 toxicities were diarrhea (29% of patients) and neutropenia (22% of patients). Grade 3 or 4 diarrhea was substantially more prevalent in the initial 18 patients on study, with an incidence rate of 56%; a significant reduction in the incidence of severe diarrhea to 9% was noted with strict adherence to an antidiarrheal regimen of loperamide and diphenyldramine. No correlations were seen between pharmacokinetics of irinotecan/SN-38 and the clinical parameters of response, survival, or incidence of diarrhea.'] Is irinotecan an active agent in untreated patients with metastatic colorectal cancer?