Severe acquired aplastic anemia (SAA) has a poor prognosis if untreated. Bone marrow transplantation is available to only a minority of patients due to lack of a matched sibling donor, advanced age of the patient, or cost. Clinical studies at NIH and elsewhere have demonstrated excellent response rates and improved survival with immunosuppressive treatments. Laboratory data implicate underlying cytotoxic T-lymphocyte-mediated suppression of hematopoiesis as the likely proximal cause of disease in most patients. In earlier clinical protocols we treated SAA with cyclosporine A (CSA) (86-H-0007), antithymocyte globulin (ATG) (87-H-0124), and combined ATG and CSA (90-H-0146). While intensive immunosuppression is most effective, relapse is common and some patients also develop second hematologic complications like myelodysplasia. In this protocol, we modify our regimen by delaying the introduction of cyclosporine to promote ATG tolerizing effects and adding mycophenolate mofetil (MMF), a new agent that, like ATG may be relatively specific for activated lymphocytes, in an effort to reduce the high relapse rate.