Many bone marrow failure syndromes in humans are now recognized to result from immunological mechanisms. These diseases include aplastic anemia, pure red cell aplasia, and some types of myelodysplasia. Patients with these conditions, who may suffer variable degrees of anemia, leukocytopenia, and thrombocytopenia, alone or in combination, have been shown to respond to a wide variety of immunosuppressive agents, ranging from corticosteroids to cyclosporine (CSA) and antithymocyte globulin (ATG), however, nonresponse and relapse continues to be a problem. Why some patients do not respond initially or others respond and then relapse is unclear. Autoreactive T cells may be resistant to the effect of ATG/CsA (nonresponders), while in others residual autoreactive T cells expand post-treatment leading to hematopoietic stem cell destruction and recurrent pancytopenia (relapse). Therefore, novel, less toxic immunosuppressive regimens that increase response rates and hematologic recovery and decrease relapse rates are needed.