A phase II study of acute leukemia with mitoxantrone (MIT) was conducted by the Tokai Blood Cancer Study Group. The drug efficacy was evaluable in 31 cases (14 of ALL and 17 of ANLL) out of 37 entered at 13 institutions. Five cases were for first induction remission and 26 cases for re-induction remission; ages ranged from 7 to 69 (median: 43); 18 males and 13 females. MIT dosage was intravenous injection of 3-6 mg/m2/day X 5 consecutive days as a rule. Of the 14 cases of ALL, 2 achieved CR and 3, PR; the efficacy rate was 36%. Of the 17 cases of ANLL, 4 achieved CR and 3, PR: the efficacy rate was 41%. Of the 5 first induction remission cases, 3 achieved CR, and 1, PR, the efficacy rate being 80%, whereas out of the 26 re-induction remission cases, 3 achieved CR, 5 PR, and the efficacy rate was 31%. In 3 of 6 cases of CR, large cumulative doses of anthracyclines such as DNR 140 mg plus ACR 410 mg, DNR 360 mg plus ADR 120 mg plus ADR 120 mg, and DNR 240 mg plus ADR 540 mg, had been administered previously in each case. As to complications, sepsis and other infections were observed at the rates of 15% and 32%, respectively, from which it was inferred that in therapy with mitoxantrone, leukopenia should be observed carefully. The major non-hematological toxicity was gastrointestinal symptoms, but the degree was mild. From the results of this trial, it was concluded that mitoxantrone was an effective form of therapy for acute leukemia. Further clinical trials on mitoxantrone in combination with other drugs are scheduled.