Hematopoietic growth factors, particularly granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, can be used as supportive agents to enhance bone marrow recovery after the administration of myelosuppressive chemotherapy given for nonmyeloid neoplasms. The use of these agents in the treatment of acute myeloid leukemia and myelodysplastic syndrome poses novel opportunities and challenges due to their direct effects on the neoplastic cells, which represent the transformed counterparts of normal hematopoietic stem cells. The interaction between hematopoietic growth factors and leukemic progenitor cells bearing a specific receptor for a given agent would be expected to result in proliferation, although maturation induction could occur. Hematopoietic growth factors have been employed as primary differentiating agents in myelodysplastic syndrome and as supportive agents after chemotherapy in acute myeloid leukemia. In either case, close monitoring for evidence of leukemic stimulation is required. Alternatively, pretreatment with colony-stimulating factors could induce cell cycling, thereby making the leukemic cells more susceptible to S-phase-specific chemotherapeutic agents, such as cytarabine.