An early phase II study of a combination regimen consisting of Idarubicin and Cytarabine was performed in patients with acute myelogenous leukemia. Idarubicin at a dose of 10 mg/m2 or 12 mg/m2 was given by 5-min. iv bolus, once daily for 3 consecutive days and Cytarabine at a dose of 80 mg/m2 was given by 2-hour iv infusion, twice daily for 7 consecutive days. Of 19 patients treated, 17 were evaluable for responses. The patients who showed a response (complete or partial remission) were 4 of 10 (40.0%) in the 10 mg/m2 group, 6 of 7 (85.7%) in the 12 mg/m2 group, and in total, 10 of 17 patients (58.8%), respectively. Analysing relationship between prior chemotherapy and responses, all of 3 patients without pretreatment, 6 of 10 patients (60.0%) in the first relapse, and 1 of 4 patients (25.0%) whose disease had been refractory to induction therapy after relapse responded. Seven of 14 patients (50%) who had received prior anthracycline therapy, showed a response. The principal adverse effects were gastrointestinal symptoms, alopecia, fever and infection, similarly to those observed in Idarubicin single therapy. When combined with Cytarabine, Idarubicin increased the incidence and severity of gastrointestinal symptoms. Considering effectiveness and safety, the clinical optimal dosage of Idarubicin when used concomitantly with Cytarabine was judged to be 12 mg/m2 once daily for 3 consecutive days.