11 patients with refractory acute leukemia of childhood were treated with idarubicin per os. Bone marrow toxicity which was observed at a dose level of 60 mg/m2 p.o. (3 x 20 mg q 24 hrs p.o.) per 3 weeks was found to be the dose limiting factor. In contrast to the first phase I study of Tan et al. (16) the maximal tolerated dose in the present study was found to be lower at a level of 90 mg/m2 p.o. (3 x 30 mg/m2 p.o. q 24 hrs) per 3 weeks. Therefore, we recommend a dosage of 60 mg/m2 p.o. (3 x 20 mg/m2 p.o. q 24 hrs) per 3 weeks as a starting dose for phase II/III studies. 2 out of the 11 anthracycline pretreated patients (91-880 mg/m2) with acute leukemia reached a complete remission undergoing idarubicin p.o. as a single therapy.