9-Nitrocamptothecin (9NC) is a water-insoluble topoisomerase I inhibitor with a broad antitumor activity in animal models. To determine the maximum tolerated oral dose (MTD), a phase I study was performed in patients with advanced cancer refractory to conventional chemotherapy. 9NC was administered orally with escalating doses to cohorts of five patients beginning at 1 mg/m2/day for five consecutive days every week for 4 weeks. Increments were 0.5 mg/m2/day for each cohort. Toxicity was evaluated in 28 patients diagnosed with various malignancies. Seven patients received 1 mg/m2/day for 28 weeks; 10 patients, 1.5 mg/m2/day for 68 weeks; and 26 patients, 2 mg/m2/day for 159 weeks. At 1.5 mg/m2/day or higher, the dose-limiting toxicity was hematologic, with grade 4 anemia in eight (29%); neutropenia in seven (25%) and thrombocytopenia in five (18%). Grade 2 or higher toxic effects occurred at each dose level: nausea and vomiting in 15 (54%), diarrhea in nine (32%), chemical cystitis in seven (25%), neutropenic sepsis in six (21%) and weight loss in five (18%) (N=28). Responses were observed after 2-8 weeks of therapy in five patients with pancreatic, breast, ovarian and hematologic tumors. Fourteen patients had a disease stabilization and one patient received treatment up to 18 months. The MTD of 9NC given orally has been estimated at 1.5 mg/m2/day for five consecutive days weekly. 9NC may be tolerated for sustained periods of time, but has the potential for significant hematologic, gastrointestinal and urinary bladder toxicity. Significant antitumor activity was observed, warranting further clinical investigations.