The colony-stimulating factors (CSFs) are polypeptide hormones that regulate myeloid proliferation and differentiation. Granulocyte-macrophage CSF (GM-CSF) has been used in leukopenic acquired immunodeficiency syndrome (AIDS) patients, myelodysplastic syndrome patients, and cancer patients after intensive chemotherapy. Its hematologic activity results in an increase in neutrophil, monocyte, and eosinophil counts. Recombinant GM-CSF effectively restores granulopoiesis in AIDS patients and appears capable of improving myelopoiesis and, in some cases, erythropoiesis in subjects with myelodysplastic syndrome. It accelerates bone marrow recovery after chemotherapy. Similarly, granulocyte CSF has demonstrated the capacity to reduce the granulocyte nadir after single agent or combination chemotherapy. Thus, CSFs have considerable promise for therapy for bone marrow failure states.