Irinotecan (CPT-II) is a semisynthetic derivative of camptothecin. It and other camptothecin analogues/derivatives appear to exert their antitumour activity by binding to topoisomerase I. The active metabolite of irinotecan, 7-ethyl-10-hydroxycamptothecin (SN-38), has demonstrated potent growth inhibition of human colorectal cancer cells in vitro, with superior activity to fluorouracil. In phase II clinical studies in patients with advanced colorectal cancer, objective response rates after irinotecan therapy ranged between 20.5 and 32%. These studies used a range of irinotecan regimens including 350 mg/m2 once every 3 weeks (Europe), 125 to 150 mg/m2 once a week for 4 weeks followed by a 2-week drug-free interval (US) and 100 mg/m2/week or 150 mg/m2 every 2 weeks (Japan). The median duration of response ranged between 5.6 and 10.6 months. Disease stabilisation occurred in 30 to 71.2% of patients. Objective response rates to irinotecan therapy in patients who had received no prior chemotherapy were similar to those in patients pretreated with fluorouracil. Importantly, irinotecan also induced responses in some patients with tumours refractory to fluorouracil. Severe (grade 3 or 4) neutropenia and diarrhoea, which occurred in up to 40% of patients receiving irinotecan therapy in phase II studies, require careful monitoring and appropriate management. Thus, irinotecan is a valuable agent for the second-line treatment of patients with advanced colorectal cancer who fail to respond to or relapse after fluorouracil therapy.