Thirty-seven children with acute leukemia were treated with 5-azacytidine in 5-day courses given every 14 days. Six out of 14 children with acute myelogenous leukemia who were adequately treated achieved an M1 marrow. Five of these subsequently developed complete remissions lasting 8 mo, 6 mo, 3 mo, 2 mo, and 2 mo. Of 22 children with acute lymphocytic leukemia, one achieved an M1 marrow and one an M2 marrow. The former attained a complete remission which lasted 3 mo. The maximum tolerated dose is between 150 to 200 mg/sq m on a daily x 5 schedule given every 14 days. The impressive activity of 5-aza-C in patients with acute myelogenous leukemia resistant to cytosine arabinoside indicates that this drug will become an important addition to the therapeutic armamentaria against this type of leukemia.