From January 1992 to July 1993, 28 patients with metastatic breast cancer were entered in a phase II trial to assess the activity and toxicity of the combination of mitoxantrone, 5-fluoruracil, and leucovorin. Patients were eligible if they had progressive disease after either adjuvant (2 patients) or previous chemotherapy for metastatic disease (26 patients). Twenty-five patients (89.2%) had received previous anthracycline-based therapy. Predominant site of metastatic disease was visceral in 22 patients, bone in 2 patients, soft tissue in 4 patients, and the majority of patients (89.2%) had two or more sites of disease. The regimen was administered according to the following schedule: Mitoxantrone 9-12 mg/m2 i.v. on day 1; L-Leucovorin 150 mg i.v. over 1 hour before 5-Fluorouracil 350 mg/m2 i.v. push days, 1, 2 and 3. Courses were repeated every 21 days. Twenty-six patients were evaluable for response. We observed 2 complete responses, 5 partial responses with a median duration of 38 weeks (range 23-68). The objective response rate was 27% (95% C.I., 10% to 44%). Myelo-suppression was the most frequent toxicity, but it was mild in the majority of patients. Nine episodes of fever and neutropenia occurred in six patients but none of these episodes was fatal. No clinical evidence of cardiotoxicity was observed. At a median follow-up of 78 weeks, the median time to progression was 20.5 weeks and the median overall survival was 48 weeks. We conclude that this regimen is well tolerated and in our experience the objective response rate is similar to other salvage chemotherapy regimens.