Aplastic anemia is characterized by decreased bone marrow function with inadequate production of erythrocytes, granulocytes, and platelets. Marrow failure may be caused by absence of or defects in hematopoietic stem cells, abnormalities of the bone marrow microenvironment, ineffective cell-to-cell interactions, or immune disorders. Although most patients with aplastic anemia have normal immunity, some have abnormalities of T- and B-lymphocytes. Rare patients have an immune cause of marrow failure. Treatment of aplastic anemia involves blood transfusions and withdrawal of potential causal factors. Efforts to stimulate hematopoiesis with androgens, corticosteroids, and other drugs have been largely unsuccessful. Some patients may recover after treatment with antithymocyte globulin or other immunosuppressive agents. Bone marrow transplantation is the preferred treatment for patients with severe aplastic anemia who have a human-leukocyte-antigen-identical related donor. Transplants of hematopoietic stem cells obtained from alternative sources, such as fetal liver cells or stem cells from long-term, in-vitro cultures, also may be useful.