A phase I study was conducted with oral irinotecan given daily for 14 days every 3 weeks in 45 patients with solid tumours to establish the maximum tolerated dose (MTD), toxicity, preliminary antitumour response and pharmacokinetics. Irinotecan was administered orally as a powder-filled capsule at doses ranging from 7.5 to 40 mg/m2 per day. Tumours were predominantly colorectal (30) together with 10 other gastrointestinal, 2 breast, 2 small cell lung and 1 ovarian. All but three patients had received prior chemotherapy. The median number of administered cycles was 3 (range 1-19). Gastrointestinal toxicities (grade 3 nausea, grade 3/4 vomiting and diarrhoea) and one incidence of grade 3 asthenia were dose limiting. There were no grade 3/4 haematological toxicities. The MTD was 30 mg/m2 per day. There were two documented partial responses, one in a patient with cancer of the small intestine and the other in a patient with colon cancer. Stable disease was seen in 16 patients (35.5%). Peak concentrations of irinotecan and metabolite SN-38 were reached within 2.0-2.4 h. The metabolic ratio of SN-38 AUC to irinotecan AUC was 0.17+/-0.10 (mean+/-SD). The dose recommended for phase II studies is 30 mg/m2 per day administered daily for 14 days every 3 weeks.