A prospective randomized study was conducted comparing the efficacy and toxicity of two anthracyclines for the treatment of patients with acute myeloid leukemia (AML) between 55 and 75 years. A total of 220 patients were randomized to receive as induction chemotherapy cytosine arabinoside (Ara-C: 100 mg/m2/day; continuous infusion for 7 days) combined with either daunorubicin (DNR: 50 mg/m2/day, i.v. bolus for 3 days) (n=108) or idarubicin (IDA: 8 mg/m2/day, i.v. bolus for 5 days) (n=112). The complete remission (CR) rate was similar (P=0.296) after IDA (76/112; 68%) and DNR (66/108; 61%) (P=0.3). For patients aged 55-65, the CR rate was significantly higher after IDA (39/47; 83%) than after DNR (29/50; 58%) (P=0.007). Persistent leukemia was more frequent after DNR (26/108) than after IDA (13/112; P=0.015). Hematological and extra-hematological toxicities were similar. The CR patients were given a consolidation course of chemotherapy with Ara-C: 50 mg/m2/12 h, subcutaneously for 5 days, combined with either DNR:30 mg m2/day, i.v. bolus for 3 days or IDA:8 mg/m2/day i.v. bolus for 3 days according to the initial randomization, and then received a continuous maintenance treatment for 2 years. The survival and disease-free survival (DFS) were similar in both groups; there was no difference in the risk of relapse. However, there was a trend for a longer event-free survival (EFS) in the IDA group than for the DNR patients (P=0.07). Our results seem to indicate that IDA is probably more efficient than DNR for AML patients between 55 and 75 years, and confirm the data published in other studies comparing prospectively IDA and DNR in adults.