The treatment of adult patients greater than 55 to 70 years of age with acute myelogenous leukemia (AML) is associated with a treatment-related mortality of approximately 25%. This prospective, double-blind randomized study was designed to see if the use of granulocyte- macrophage colony stimulating factor (GM-CSF; yeast-derived) could shorten the period of neutropenia and to determine any effect this would have on therapy-related morbidity and mortality. A total of 124 patients entered this study. Induction consisted of standard daunorubicin and cytarabine. A day-10 bone marrow was examined; if this was aplastic without leukemia, patients received blinded placebo or GM- CSF from day 11 until neutrophil recovery. Patients who entered complete remission received the identical study medication (blinded GM- CSF or placebo) in consolidation that they had received during induction. The overall complete remission rate was 52%; 60% for the GM- CSF arm and 44% for the placebo arm (P = .08). Median times to neutrophil recovery were significantly shortened on the GM-CSF arm. The overall treatment-related toxicity from start of GM-CSF/placebo was reduced on the GM-CSF arm (P = .049). Similarly, the infectious toxicity was significantly reduced on the GM-CSF arm (P = .015). The median survival for all patients was 10.6 months in the GM-CSF group and 4.8 months in the placebo arm (P = .048). It appears that GM-CSF is safe and efficacious for adult patients greater than 55 to 70 years of age with AML; its major impact is in reducing the duration of neutropenia and therapy-related mortality and morbidity. This may result in a better response rate.