Mitoxantrone, a new anthracenedione, was administered to thirty-nine patients with relapsed and refractory acute leukemia and to 12 patients with blastic crisis of chronic myelogenous leukemia between August 1981 and September 1984. Eleven patients were not evaluable and 40 were analysed. There were 24 males and 16 females with a median age of 37 yrs (range 6-73 yrs). Three of these were less than 15 yrs and 7 more than 60 yrs. The initial dose employed was 1.9 mg/m2/day X 5. Although eventually a starting dose of 12.3 mg/m2/day X 5 was used, about one half of cases were given more than 5 mg/m2/day X 5 by i.v. bolus. Among 25 patients with acute non-lymphocytic leukemia, there were 4 complete and 6 partial remissions. Among 7 patients with acute lymphocytic leukemia there was one complete remission and one partial remission. All patients except one who attained remissions had received prior anthracyclines. One of 8 patients with blastic crisis of chronic myelogenous leukemia had a partial remission. The durations of complete remission were 1, 1, 5+, 13+ and 17 weeks, respectively. Side-effects showed expected bone marrow depression. Mucositis occurred in ten patients. Gastrointestinal symptoms were noted in approximately 50%, but were mostly mild. Mild alopecia occurred occasionally. The trials were too short to allow evaluation of possible cardiac toxicity. These data indicate that mitoxantrone is non-toxic but hematological and a promising single drug for use in treating relapsed and refractory acute leukemia and suggest that further study would be worthwhile in order to identify its role in the first-line therapy of acute leukemia.