As part of a broad phase I study of recombinant human granulocyte-macrophage colony-stimulating factor (rh GM-CSF), four patients were treated who had myelodysplastic syndrome (MDS) with excess blasts. The GM-CSF was given daily as an intravenous injection over a period of 30 min for 5 days. A total of 11 cycles were conducted. Each patient received at least two different dose levels. In three patients, three different dosages were delivered. The treatment course was interrupted by a 10-day rest period. Rh GM-CSF was well tolerated, with only minor side effects seen, which included bone discomfort at the lower back, sternum and ribs, and constitutional symptoms such as low grade fever, nausea/vomiting, and mild myalgias. Whereas no increases in platelet and reticulocyte counts were recorded, elevations of absolute neutrophil counts above 100 cells/microliters occurred in all patients. The most striking finding was, however, the development of increases in the number of circulating and bone marrow blast counts that were observed particularly when doses of greater than or equal to 500 micrograms/m2 of body surface area were administered. In line with data demonstrating in vitro induction of proliferation of leukemic blast cells by rh GM-CSF, one may take advantage of blastogenesis induced in vivo that may favor the use of a therapeutic strategy by recruiting quiescent cells into the mitotic cycle which would then represent optimum targets for a subsequent cycle-specific cytotoxic chemotherapy. Such an approach could form the basis for new clinical trials in MDS.