Mitoxantrone was administered at the dose of 14 mg/m2 i.v. every 3 weeks to 25 consecutive women with measurable progressive disease who relapsed after adjuvant CMF and endocrine therapy. The treatment plan consisted in the delivery of 6 cycles, unless disease progression or severe toxicity occurred. All patients were evaluable for drug response and toxicity. One patient achieved complete remission and 6 partial remission, for a total response rate of 28%. Objective tumor response was observed in all major sites of disease. The median time to achieve remission was 3 months. The median duration of response was 7 months (range, 5-39+), and the median survival for the entire group was 10 months (range, 3-39+). Results were influenced only by the duration of disease-free status from the end of adjuvant CMF chemotherapy. In fact, all tumor responses were documented in woman with free intervals exceeding 1 year (7 of 17 or 41%). Treatment was generally well tolerated, with 10 patients developing leukopenia at some time during treatment. Only 2 patients received less than 75% of the projected dose because of granulocytopenia. Complete alopecia occurred in only 2 cases. Three patients developed a fall greater than 15% in left ventricular ejection fraction, but no episode of congestive heart failure was observed. We conclude that mitoxantrone is an effective and safe drug which can be utilized in women relapsing after adjuvant CMF.