Half of our patients presenting with breast cancer will eventually require cytotoxic chemotherapy. In searching for an effective drug with low toxicity, many new drugs have been investigated. Seventeen patients with advanced breast cancer were treated with mitoxantrone, an analogue of adriamycin. The response rate was 44% (CR 6% + PR 38%). Side effects were fewer than those normally anticipated for most cytotoxic chemotherapy in that only two patients suffered hair loss and three nausea and vomiting. There was, however, significant bone marrow depression with granulocytopenia and two patients died of septicaemia following treatment. Cardiac toxicity occurred in two patients: of these, both had prior treatment with adriamycin, and one had prior radiotherapy to the chest wall. Both of these patients died.