Aplastic anemia is a disease that presents with a hypocellular marrow and peripheral blood pancytopenia. In Europe and the United States, it has an age-adjusted incidence per million population per year of 2.2 compared to 11.0 in Japan and Korea. Pathogenic mechanisms are varied and include intrinsic defects of hematopoietic stem cells, defects in the marrow microenvironment, and abnormal humoral or cellular immune control of hematopoiesis. In most patients, aplastic anemia is of unknown etiology, whereas in some, the disease can be related to infections, drugs and chemicals, and hereditary causes. Therapy for aplastic anemia includes blood component transfusions, antibiotics, androgenic steroids, and corticosteroids. With supportive care, most patients with aplastic anemia die within a year of diagnosis, and only approximately 20% of patients are surviving, although often with persisting hematologic abnormalities. The use of hematopoietic growth factors has shown, for the most part, only transient beneficial effects. More definitive therapy has been the use of immunosuppressive agents including antithymocyte globulin, cyclosporine, and cyclophosphamide. With immunosuppressive therapy, a variable proportion of patients respond to therapy, ranging from 20% to 80%. However, although responses may be frequent, long-term outlook is guarded because some patients may relapse with aplastic anemia, whereas others may go on to have a clonal disorder develop, including myelodysplasia, leukemia, or paroxysmal nocturnal hemoglobinuria. As a result, survival estimates at 15 to 18 years may be only on the order of 30%. More definitive therapy has been with transplants of hematopoietic stem cells from allogeneic donors. Transplants are carried out after high-dose immunosuppressive conditioning programs. Best current results show long-term, event-free survivals with successful allografts on the order of 90%.