As phase II study, we treated 18 patients with myelodysplastic syndrome (MDS) and 37 patients with aplastic anemia (AA) with recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) for 14-28 days. Administration of rhGM-CSF resulted in a dose-dependent increase in circulating granulocyte counts, which was statistically significant in patients with AA. There were no consistent changes in monocyte and lymphocyte counts. Although no increase in both thrombocyte and erythrocyte counts was detected in the majority of the patients, a response of both lineages to rhGM-CSF, in addition to granulocyte lineage, was observed in 3 patients. Drug-associated adverse events developed in 28 patients (51%). The most frequent adverse event was fever. In general, the treatment with rhGM-CSF was well tolerated. The results suggest that rhGM-CSF is effective for patients with MDS and AA.