Twenty-six patients affected by acute leukemia were treated with 4-demethoxydaunorubicin (idarubicin), a new anthracycline compound which in experimental leukemias showed an antitumoral activity superior to daunorubicin (DNR) and doxorubicin (DX), with a higher ratio of active to cardiotoxic doses. A group of 16 patients in relapse received idarubicin at a dosage of 5-6 mg/m2/day for 3 consecutive days; a second group of 6 relapsing and 4 previously untreated cases was treated with a sequential combination of idarubicin and arabinosyl cytosine. In all patients, a significant fall of bone marrow and peripheral blast cells was obtained. These preliminary results suggest that idarubicin has a therapeutic activity against human acute leukemias usually responsive to DNR or DX. The duration of myelosuppression varied from 7 to 50 days, leading in some cases to a high risk of infections. As regards other toxic effects (gastrointestinal, hepatic and acute cardiac toxicity, alopecia), idarubicin appears to be, in our experience, a well-tolerated drug; however, it is too early to comment on delayed cardiac effects.