Four patients with very severe aplastic anemia refractory to antilymphocyte globulin were administered recombinant human granulocyte-macrophage--colony stimulating factor (GM-CSF). One patient with minimal residual myelopoiesis responded transiently to two separate courses of GM-CSF at 4 and 8 micrograms/kg/d administered intravenously and another course at 4 micrograms/kg/d administered subcutaneously. Septicemia and bilateral pneumonia that had been resistant to conventional therapy resolved. Three patients with no evidence of residual myelopoiesis did not respond to GM-CSF. In one patient, the dose was increased to 32 micrograms/kg/d with no effect on hematopoiesis. Immediate side effects were minimal at GM-CSF doses up to 16 micrograms/kg/d. GM-CSF may, however, have been involved in the pathophysiology of thrombosis of the inferior vena cava in the patient administered 32 micrograms/kg/d. We conclude that GM-CSF does not induce hematopoiesis in long-standing, severe, treatment-resistant aplastic anemia with complete myelopoietic failure. However, in patients with minimal residual myelopoiesis, GM-CSF could be a promising adjuvant therapy for severe infection.