BACKGROUND: Irinotecan is active against colorectal cancer in patients whose disease is refractory to fluorouracil. We investigated the efficacy of these two agents combined for first-line treatment of metastatic colorectal cancer. METHODS: 387 patients previously untreated with chemotherapy (other than adjuvant) for advanced colorectal cancer were randomly assigned open-label irinotecan plus fluorouracil and calcium folinate (irinotecan group, n=199) or fluorouracil and calcium folinate alone (no-irinotecan group, n=188). Infusion schedules were once weekly or every 2 weeks, and were chosen by each centre. We assessed response rates and time to progression, and also response duration, survival, and quality of life. Analyses were done on the intention-to-treat population and on evaluable patients. FINDINGS: The response rate was significantly higher in patients in the irinotecan group than in those in the no-irinotecan group (49 vs 31%, p<0.001 for evaluable patients, 35 vs 22%, p<0.005 by intention to treat). Time to progression was significantly longer in the irinotecan group than in the no-irinotecan group (median 6.7 vs 4.4 months, p<0.001), and overall survival was higher (median 17.4 vs 14.1 months, p=0.031). Some grade 3 and 4 toxic effects were significantly more frequent in the irinotecan group than in the no-irinotecan group, but effects were predictible, reversible, non-cumulative, and manageable. INTERPRETATION: Irinotecan combined with fluorouracil and calcium folinate was well-tolerated and increased response rate, time to progression, and survival, with a later deterioration in quality of life. This combination should be considered as a reference first-line treatment for metastatic colorectal cancer.