Twenty-two adult patients with relapsed leukemia were given aziridinylbenzoquinone (AZQ) intravenously in a phase I clinical trial. AZQ was administered as a 30-minute infusion daily for 17 days. Courses were repeated when the bone marrow returned to normal cellularity and full recovery from nonhematologic toxicity had occurred. The initial dose of AZQ was 8 mg/m2/d x 7. The highest dose given was 28 mg/m2/d x 7. A maximum of three patients were treated at each dose level and patients received at least two courses at a given dose level before they were eligible to be escalated to the next higher available AZQ dose level. Nonhematologic side effects were mild and included nausea/vomiting (32%), mucositis (18%), and alopecia (0%). Dose-limiting toxicity was bone marrow aplasia at the 28 mg/m2/d x 7 level. No complete or partial responses were observed in this initial study. For phase II adult leukemia studies using this schedule, it is recommended that the AZQ dose should be 24 mg/m2/d.