Patients (n = 174) with metastatic breast cancer previously untreated with anthracycline cytotoxic agents were randomized into two groups: Group 1 received FEC (5-fluorouracil 500 mg/m2, epirubicin 60 mg/m2 and cyclophosphamide 500 mg/m2) once every fourth week and group 2 received the treatment once weekly in the same monthly dosage. Treatment was recommended to continue until disease progression or to a cumulative epirubicin dose of 1,000 mg/m2, but could be discontinued at any time at the patient's request or at the treating physician's judgement. An interim analysis was made when 131 patients were evaluable for response, and 128 patients for toxicity. Hematological toxicity was significantly more severe in the monthly group, as was nausea and vomiting. Of the monthly treated patients 76% had total alopecia compared to 14% in the weekly group. There were no statistically significant differences in the occurrence of mucositis. Monthly FEC gave significantly higher response rate than weekly treatment (52 vs 34%, p = 0.01). Time to progression was significantly (p = 0.004) longer with monthly FEC. Patients in the monthly treated group lived significantly (p = 0.02) longer than patients in the weekly group. These results indicate that both toxicity and efficacy of epirubicin-containing combination therapy in breast cancer is dependent on the treatment schedule, not merely on dosage. Both efficacy and toxicity increased when the treatment was given once monthly compared to the weekly schedule.