Mitoxantrone is a new anthracenodione derivative with a high antineoplastic activity in proliferative diseases of the haemopoietic system. In the Institute of Haematology in Warsaw and in the Department of Haematology, Silesian Medical Academy in Katowice this agent was used in combination with cytarabine in 49 cases of acute leukaemia (35 with acute myeloid leukaemia and 14 with acute lymphoblastic leukaemia). The preparations used were Mitoxantrone (POLFA Works in Jelenia Góra) and Novantrone (Lederle). These agents were given intravenously in doses of 10-20 mg/m2 for 3 days in combination with cytarabine in three doses: 100 mg/m2 on days 1 through 7, and 1 g/m2 or 3 g/m2 every 12 hours on days 1 through 4 of the treatment. Complete remission was obtained in 17 cases (35%), including 13 with acute myeloid leukaemia (37%) and 4 with acute lymphoblastic leukaemia (29). The most frequent side effects were: long-lasting pancytopenia (in 100% of cases), hair loss (39%) and gastrointestinal toxicity (33%). No significant differences were noted in the effectiveness and toxicity between these two preparations. In the light of the presented results it may be accepted that the combination of mitoxantrone with cytarabine is an important advance in the treatment of acute leukaemias.