Aclarubicin is a new anthracycline antibiotic that produces substantially less cardiotoxicity in animals that does doxorubicin. Based upon prior Phase I and II trials in leukemia, a Phase II study in acute myeloblastic leukemia was developed to assess the response rate and toxicity in previously treated patients. Forty patients received aclarubicin 100 mg/m2 per day X 3 with repeated course on days 14-16 if marrow hypoplasia was not produced. Complete responses were achieved in 27.5% (11/40) with durations of 1.5, 2, 2, 2, 3, 3+, 4, 5+, 32+, 33+, and 34+ months. Toxic effects of this therapy included severe neutropenia and thrombocytopenia, nausea/vomiting, mucositis, and diarrhea. No patient developed significant changes in the left ventricular ejection fraction, as measured by radionuclide angiography, or any clinical cardiac symptoms. Alopecia was minimal. Aclarubicin can produce a significant response rate in previously treated patients with acute myeloblastic leukemia and should be considered for study in initial therapy.