In this study 523 previously untreated patients with acute myelocytic leukemia were randomly allocated to induction therapy with daunorubicin 60 mg/M2 daily X 3, cytosine arabinoside and thioguanine 100 mg/M2 each every 12 hours until marrow hypoplasia was achieved, or a 5-day course of the three drugs with daunorubicin 100 mg/M2 given on dav 1 and cytosine arabinoside plus thioguanine each given at a dose of 100 mg/M2 every 12 hours for five days. All patients received cyclophosphamide 600 mg/M2 followed in 24 hours by hydroxyurea 500 mg/M2 every six hours for four doses monthly for maintenance therapy. Patients were randomized to receive one of three antimetabolite treatments beginning 24 hours after the last dose of hydroxyurea each month for seven days. One such treatment consisted of 6-mercaptopurine 100 mg/M2 daily, another group received 6-thioguanine at the same dose daily, and the third group received 50 mg/M2 of both antimetabolites daily. There were no significant differences in complete response rate, remission duration, or survival among the various treatment groups.