The purpose of this prospective study was to assess the efficacy, clinical benefit and safety of irinotecan (CPT-11) in patients with 5-fluorouracil-resistant metastatic colorectal cancer (CRC). Sixteen patients with World Health Organization (WHO) performance status < or = 2 were treated with CPT-11 350 mg/m2 every 3 weeks. The observed partial response (PR) rate was 6.3 per cent with a high rate of stable disease (SD) (43.7%) which was of long duration (21.1 weeks for the best response; 1PR, 7SD). The median survival time for the 16 patients entered into this study was 69.6 weeks. There was no toxic death. The most frequent adverse events were neutropenia (31% grade 3/4) and delayed diarrhea (9.7%). CPT-11 has definite activity in the treatment of progressive metastatic CRC truly resistant to 5-FU which translated into clinical survival benefit. Median survival from first administration of CPT-11 was 78.6 weeks for patients with best response (PR + SD) compared with 28.1 weeks for patients with progressive disease (PD) (P = 0.01). With the appearance of new active drugs in this highly chemotherapy-resistant disease, the definition of response should be reassessed in CRC.