This randomised clinical trial, involving patients with advanced colorectal cancer, was carried out to compare the effectiveness of accelerated folinic acid (FA) plus 5-fluorouracil (5-FU) with that of the conventional regimen of 5-FU alone. Both regimens were administered with simulataneous supportive care. 185 patients were eligible: 94 were randomly allocated to receive FA 200 mg/m2 i.v. plus 5-FU 400 mg/m2 i.v. on days 1-5 every 3 weeks; and 91 to receive 5-FU 400 mg/m2 i.v. on days 1-5 every 4 weeks. The response rate was 33.3% in the accelerated FA/5-FU and 18.6% in the 5-FU arm (P = 0.045). Median survival was 13.5 months in the FA/5-FU arm and 7.5 months in the 5-FU arm (P = 0.039). Toxicity was mild and slightly more pronounced in the FA/5-FU arm (P = 0.078). This study indicates that, in patients with advanced colorectal cancer, accelerated chemotherapy with FA and 5-FU and simultaneous supportive care is capable of achieving a higher response rate and longer survival than conventional 5-FU alone, without severe toxicity.