A phase II trial was performed to assess the efficacy and toxicity of the combination mitoxantrone (MXN) and vinorelbine (VNR) as first-line chemotherapy for metastatic breast cancer. Forty-one patients with metastatic disease or local relapse recruited between March 1991 and April 1993 received a first-line chemotherapy treatment consisting in 12 mg/m2 intravenous (IV) bolus of MXN on day 1 followed by a 20-minute perfusion of 25 mg/m2 of VNR on days 1 and 8. Cycles were repeated every 21 days until evidence of disease progression or of severe toxicity. Thirty-seven patients were evaluable for response and all 41 for toxicity. An objective response was observed in 19 patients (51%; 95% confidence interval, 45 to 74%). The response was complete in a further 11 (30%). Median time to treatment failure was 9 months. Median survival was 14 months. There were no treatment-related deaths. Limiting toxicity was myelosuppression. Leukopenia occurred in 29 patients (71%) and was grade 3 or 4 in nine of these (15%). Grade 2 or 3 anemia was encountered in six patients (15%), grade 1 thrombocytopenia in one, neurotoxicity (constipation) in two, and grade 2 or 3 alopecia in 12 (29%). Nausea/vomiting requiring antiemetic treatment was experienced by only two patients (5%). There were two cases of septicemia treated by antibiotic therapy in hospital.