Forty-two women with measurable or evaluable advanced breast cancer who had received neither prior chemotherapy for advanced disease nor any anthracycline-containing regimen as adjuvant were entered in a phase II study of mitoxantrone (Novantrone; dihydroxyanthracenedione). Patients were aged from 36 to 80 years, performance status was from 0 to 2. All patients had normal hematological status and normal renal and liver function tests. Cardiac scintigraphy and sonography techniques were used to monitor cardiac function. Mitoxantrone was administered at a dose of 14 mg/m2 in 100 ml 5% dextrose solution over 30 minutes, repeated every three weeks. The number of courses per patient ranged from 2 to 12. Of 42 eligible patients 39 were fully evaluable for response and all for drug toxicity. Responses to treatment were: complete response four patients, partial response 10 patients, stable disease 18 patients and progressive disease seven patients. The overall response rate was 36% (95% confidence limits 20-52%). Three patients showed decreased left ventricular ejection fraction but no patient developed signs of overt left ventricular failure during the treatment period. Hematological and gastrointestinal toxicities were mild. Hair loss was minimal. The data indicate that mitoxantrone is an effective agent for the treatment of advanced breast cancer with mild side-effects, especially with respect to nausea/vomiting, hair loss and cardiotoxicity.