Aclarubicin (ACR) was administered in a prospective cooperative phase II trial to 44 patients with possibly refractory acute nonlymphocytic leukemia who were previously treated with daunorubicin and cytarabine. Induction treatment consisted of 80 mg/m2 of ACR iv daily for 3 days, followed by 80 mg/m2 iv daily for 2 days in patients not obtaining a complete remission (CR) after 2-4 weeks. CR was observed in eight patients (18%) and partial remission was observed in six (14%). On monthly maintenance chemotherapy with ACR and cytarabine, the duration of CRs varied between 10 and 58 weeks. Achievement of remission was not related to age, presence or absence of Auer bodies, cytogenetic characteristics, or previous response to daunorubicin and cytarabine. Side effects were nausea and vomiting observed in 86% and diarrhea in 34% of the patients, whereas mucositis and alopecia were uncommon. Disturbances of cardiac function arousing suspicion of acute ACR toxicity were observed in seven patients. No case of chronic cardiotoxicity was observed, despite the fact that 20 patients received ACR doses greater than 400 mg/m2, with seven of the 20 having had a previous daunorubicin dose greater than 400 mg/m2. As CR was obtained in four of 14 patients with primary therapy-resistant leukemia and in two of 16 patients with relapse and no response to re-treatment with daunorubicin and cytarabine, ACR does not seem to show clinical cross-resistance to daunorubicin. Evaluation of ACR in first-line chemotherapy of acute nonlymphocytic leukemia appears justified.