Eleven academic institutions were selected to study mitoxantrone administered on a schedule of 10 mg/m2/d for five days initially and later at 12 mg/m2/d for five days, each given as a 30 minute intravenous (IV) infusion each day. Patients with acute or chronic leukemia were stratified by leukemic type and clinical status and included one group of patients considered to be in relapse after complete remission from previous chemotherapy and another group of patients considered refractory to standard induction and/or salvage chemotherapy. During the initial treatment schedule, complete remissions were obtained in two of seven patients with acute nonlymphoblastic leukemia, in one of three patients with acute lymphoblastic leukemia, but in none of the patients with chronic granulocytic leukemia in blast crisis. The durations of remission for these three patients were 22, 57, and 78 days, respectively. An increase in mitoxantrone dose to 12 mg/m2/d produced complete remissions in 8 of 19 evaluable patients with acute nonlymphoblastic leukemia, in one of ten patients with refractory acute nonlymphoblastic leukemia, and in one of four patients with chronic granulocytic leukemia in blast crisis. Each of these patients required only a single course of mitoxantrone to achieve remission; the median time to remission was 37 days (range 18 to 64 days). Remission duration ranged from 35 days (chronic granulocytic leukemia) to 186 days, with the median duration for those patients with acute nonlymphoblastic leukemia achieving remission being 135 days. Of the six patients with acute lymphoblastic leukemia, none achieved remission at the higher dose level. Drug-related gastrointestinal toxicity included mucositis (25%), diarrhea (21%), and nausea and vomiting (61%). Systemic infection (nonfatal) was experienced by 21% of patients and alopecia by 17%. Other side effects that occurred occasionally were hepatic dysfunction, decreased renal function, confusion, lethargy, anxiety, and fever. Possible drug-related phlebitis developed in one patient, and a single episode of minor epistaxis was reported in another. Cardiovascular toxicity was low. At a mitoxantrone dose of 10 mg/m2/d for five days, one patient developed hypotension, and one episode of congestive heart failure was reported in another. At the higher dose of 12 mg/m2/d, no drug-related hypotension, congestive heart failure, tachycardia, or chest pain were reported. These data indicate that mitoxantrone is a promising single drug for the treatment of acute nonlymphoblastic leukemia and possibly for acute lymphoblastic leukemia.