We performed a phase I/II study of the administration of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) to patients with aplastic anemia or myelodysplastic syndrome. Doses ranging from 15 to 480 micrograms/m2 were administered as a one-hour or four-hour intravenous infusion daily for 7 days or as a 12-hour infusion for 14 days. Temporary improvements were seen in granulocyte counts, monocyte counts, and reticulocyte counts in six of eight patients with aplastic anemia and five of seven patients with myelodysplastic syndromes. The patients with myelodysplastic syndromes had larger increases in granulocyte, monocyte, and reticulocyte counts than did those with aplastic anemia, and they also had increases in the numbers of eosinophils (two of seven), immature myeloid cells (two of seven), and myeloblasts (two of seven) that were not observed in patients with aplastic anemia. There was no reduction in erythrocyte transfusion requirements, and no effect was observed on platelet counts. There was only minimal toxicity consisting of transient low-back discomfort, anorexia, myalgias/arthralgias, and low-grade fever. Our data suggest that GM-CSF is well tolerated and is more likely to result in elevations of blood counts in patients with myelodysplasia than in patients with aplastic anemia, but the role of GM-CSF therapy in these disorders remains to be determined.