Sixty-six newly diagnosed patients with acute nonlymphocytic leukemia received either daunorubicin alone or a combination of daunorubicin, cytosine arabinoside, 6-thioguanine, and pyrimethamine for remission-induction therapy. The two treatment groups were comparable with respect to the two major prognostic factors in this disease, which were age and presence or absence of infection on admission. The two therapies produced similar results with respect to CR rate and median survival results. Single-agent therapy was associated with less frequent utilization of hospital inpatient facilities and fewer platelet transfusions. The four-drug combination did not decrease the incidence of meningeal leukemia. Patients who achieved CR were treated with two half-dose consolidation courses of the successful remission-induction regimen. Subsequently, all patients received cyclophosphamide and guanazole monthly for maintenance therapy. Median durations of remission for both induction-treatment groups were similar (6.8 and 5.6 mos). The therapeutic results with the single agent in this study were not inferior to those obtained with the drug combination tested, as well as most other previously reported combinations of antileukemic drugs.