The multilineage hematological effects of the chemoprotective agent amifostine suggested administration of the drug to patients with myelodysplastic syndromes. Partial but encouraging responses were seen. The aim of this research was to further evaluate amifostine activity on the hemopoietic progenitor cells of myelodysplastic patients. We determined amifostine effects on in vitro growth of pre-treatment peripheral blood and bone marrow hemopoietic progenitor cells from myelodysplastic patients, and colony forming unit granulocyte-macrophage and burst forming unit erythroid levels in the peripheral blood of 7 myelodysplastic patients during and following a 3-week treatment with amifostine (200 mg/m2 i.v., three times a week). No variations were observed for hemoglobin, platelets, white blood cells and reticulocyte values. The peripheral blood levels of both colony forming unit granulocyte-macrophage and burst forming unit erythroid, however, showed a significant and sustained increase. In vitro a significant effect exerted by the drug was observed in single cases, but the in vitro effect did not predict the in vivo increase of peripheral blood levels of hemopoietic progenitor cells. These results appear to provide evidence that amifostine has a significant impact on early events of hemopoiesis in myelodysplastic patients.