Recombinant human granulocyte colony-stimulating factor (rhGCSF) has been commercially available for two years; yet unanswered questions need to be substantiated by clinical trials. The most extensively studied clinical application has been in chemotherapy-induced neutropenia. GCSF accelerates neutrophil recovery after bone marrow transplant, mobilizes bone marrow progenitor cells into peripheral blood harvesting then transplanted as supportive measures in patients undergoing intensive chemotherapy, and accelerates neutrophil recovery in patients with acute leukemias. GCSF is generally well-tolerated with only mild to moderate bone pain. Mild reversible elevation in lactate dehydrogenase, alkaline phosphatase, and uric acid has been reported. As data are obtained from ongoing trials, GCSF's clinical role will be expanded and better defined.