Abstract Fifteen women with metastatic breast cancer undergoing autologous bone marrow transplantation were treated with an intensive induction regimen including cyclophosphamide and mitoxantrone in a phase I-II stydy. The cardiotoxicity of these two drugs in combination have not been previously reported. Three patients developed deeply inverted T-waves during or shortly following conclusion of high-dose chemotherapy with associated declines in ejection fraction of 0.04, 0.19 and 0.27. One patient developed a brief episode of congestive heart failure which resolved with supportive care. These three patients were the only ones to have received previous left chest wall radiotherapy during the course of their disease, prior to treatment with autologous bone marrow rescue. No patient developed dysrhythmia or QRS changes during a mean hospital stay of 33.7 days (range 27–40 days).