To reduce critical neutropenia after chemotherapy (CT) for acute myeloid leukemia (AML) we administered recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) to patients over the age of 65 years with newly diagnosed AML and to patients with early or second relapse. CT was 9-day 6-thioguanine, ara-C, and daunorubicin (TAD9) in newly diagnosed AML and sequential high-dose ara-C and mitoxantrone (S-HAM) for relapse. In patients whose bone marrow was free from blasts a continuous intravenous infusion of GM-CSF 250 micrograms/m2/d started on day 4 after CT. Thirty-six patients entered the study and 30 of them did receive GM-CSF. For comparison, a historical control group of 56 patients was used. Complete remission rate was 50% (18 of 36) versus 32% in controls (P = .09), and early death rate was 14% versus 39% (P = .009). Treatment with GM-CSF was not associated with major adverse events. Two patients showed a marked leukemic regrowth that was completely reversible in one patient and appeared to be GM-CSF independent in the other patient. Remission duration does not seem to be reduced after GM-CSF. Under GM-CSF the blood neutrophils recovered 6 and 9 days earlier in the TAD9 (P = .009) and S-HAM (P = .043) groups associated with a rapid clearance of infections in most patients. We conclude that GM-CSF was of therapeutic benefit to our patients and this provides a basis for larger controlled trials.