Mitoxantrone is a substituted anthraquinone with considerable activity against human acute leukemia. The authors' goal was to treat patients with continuous infusion mitoxantrone in order to maintain cytotoxic steady state levels with acceptable toxicity and to assess the results. Daily mitoxantrone levels showed a mean steady state plasma level of 16.8 k 1.4 ng/ml (range, 9.1-25.1) with a systemic clearance of 519 k 47 ml/minute/mz. No drug accumulation occurred. Mitoxantrone was undetectable 24 hours postinfusion. All patients, including two patients with chronic myelogenous leukemia in blast phase, had > 90% reduction in leukemia cell mass (marrow cellularity X percent leukemia cells) by day 6. However, six patients received 3 days of etoposide at that point because of residual acute nonlymphocytic leukemia (ANLL). Overall four patients (36%) had a complete remission; one additional patient had a bone marrow remission but also had a persistent granulocytic sarcoma. Toxicities included severe but tolerable myelosuppression, mucositis, and hepatic dysfunction. There was no correlation between mitoxantrone levels, toxicity, or clinical response. Continuous infusion produces cytotoxic plasma mitoxantrone levels and rapid clearing of ANLL from bone marrow. Further dose escalation may be possible.