Infusional 5-fluorouracil in advanced breast cancer has been associated with improved clinical response rates when compared with conventional bolus therapy. As a first line of chemotherapy in proven metastatic breast carcinoma, 258 women were randomly assigned to receive FAC consisting of 5-fluorouracil (F) 600 mg m(-2) intravenously (i.v.) over 1 h on days 1, 2 and 3, doxorubicin (A) 50 mg m(-2) i.v. bolus on day 1 and cyclophosphamide (C), 400 mg m(-2) i.v. bolus on days 1, 2 and 3 or 'FULON' consisting of 5-fluorouracil 250 mg m(-2) day(-1) continuously infused from day 1 to day 22, doxorubicin 15 mg m(-2) i.v. bolus on days 1, 8, 15 and 22 and cyclophosphamide 300 mg m(-2) i.v. bolus on days 1, 8, 15 and 22. Chemotherapy courses were administered 4-weekly for the bolus regimen and 6-weekly for FULON. Pretreatment characteristics were identical between the two groups. Response rates were 54% in the FAC arm and 53% in the FULON arm. Time to progression was 14 months in the FAC arm and 12 months in the FULON arm. Differences were not statistically significant. Median overall survival duration for all patients was 22 months. Haematological toxicity was more severe in the bolus-treated group (P = 0.05), as were nausea and vomiting (P < or = 0.01). We conclude that the two regimens appeared equally effective but have different toxicities.