Clinical trials with hematopoietic growth factors (granulocyte-macrophage colony-stimulating factor [GM-CSF], granulocyte colony-stimulating factor [G-CSF], interleukin-3, erythropoietin] have been done in patients with myelodysplastic syndromes. Treatment with GM-CSF or G-CSF has resulted in an increase of neutrophil counts into the normal range in the vast majority of patients. Progression to acute leukemia does not appear to occur more frequently in the patients receiving GM-CSF or G-CSF. Increases in platelet counts and hemoglobin levels have been reported after treatment with interleukin-3 and erythropoietin, respectively, although the response is only seen in a minority of treated patients. Combination therapy with GM-CSF and low-dose cytosine arabinoside has been studied, but present data do not indicate an advantage over other treatment strategies. Cytogenetic and molecular genetic analyses demonstrate that both normal and malignant precursor cells are stimulated by cytokine therapy.