A comparative trial of a combination of daunorubicin (DNR) and cytosine arabinoside (ara-C) (Regimen A) and a combination of aclarubicin (ACR) and ara-C (Regimen B) was performed. Sixteen patients with acute non-lymphocytic leukemia, previously untreated, included in this study. Five of 8 patients, 62.5%, obtained a complete remission (CR) with both Regimen A and B. The days until CR varied from 37 to 46 in Regimen A and from 22 to 56 in Regimen B. The total doses of DNR and ara-C in Regimen A were from 100 to 240mg and from 640 to 1,120mg, respectively. Those of ACR and ara-C in Regimen B were from 180 to 300mg and from 660 to 1,000mg, respectively. In a comparative study of hematological changes, toxic effects on peripheral white blood cells, platelets and nucleated cells in bone marrow tended to appear later in Regimen B than in Regimen A. Side effects on the digestive system such as nausea and vomiting and vascular pain were more frequently recognized in patients treated with Regimen B, although they were managed by symptomatic treatment. The present comparative study indicated that a new antileukemic anthracycline, ACR, has the same therapeutic activity against ANLL as DNR in combination with ara-C.