In a prospective randomized study, five European transplant centers compared recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF; mammalian glycosylated) with placebo. rhGM-CSF was administered in a dose of 8 micrograms glycoprotein (5.5 micrograms protein)/kg/d, as a continuous intravenous (IV) infusion for 14 days, starting 3 hours after bone marrow infusion. Fifty-seven patients entered and completed the study. Median age of the recipients was 34 years (range, 17 to 51 y). All donors were HLA-identical, MLC-nonreactive siblings. Marrow grafts were depleted of T lymphocytes either by counterflow centrifugation (n = 42) or by immunological methods (n = 15). Twenty-nine patients received rhGM-CSF and 28 patients placebo. The leukocyte count and the absolute neutrophil count were significantly higher in the rhGM-CSF-treated group from day +9 to day +14 after bone marrow transplantation (BMT). This was also true for the monocyte count from day +12 to day +21. Early neutrophil (greater than 0.1 and greater than 0.3 x 10(9)/L) and early leukocyte (greater than 0.3 and greater than 0.5 x 10(9)/L) recovery was significantly faster for the patients given GM-CSF. The incidences of graft-versus-host disease (GVHD) and transplant-related mortality were not different in both groups. However, the number of bronchopneumonias was significantly lower in the rhGM-CSF-treated group (P = .03). Long-term follow-up showed a trend to better overall disease-free survival at 2 years and a trend to a lower relapse risk in patients treated with rhGM-CSF. This study shows that rhGM-CSF significantly increases neutrophil and monocyte counts during periods of 6 to 10 days in the second and third week after BMT. This shortened period until myeloid cell recovery after transplantation resulted in a decreased number of pneumonias, without an increase in incidence of GVHD or relapse.