Risk factors for graft failure were analyzed in 122 recipients of an allogeneic T-cell-depleted human leukocyte antigen (HLA)-identical sibling marrow transplant as treatment for leukemia. In each case pretransplant immunosuppression included 1,375 to 1,500 cGy hyperfractionated total body irradiation and cyclophosphamide (60 mg/kg/d x 2). No patient received immunosuppression prosttransplant for graft-versus-host disease (GVHD) prophylaxis. Nineteen patients in this group experienced graft failure. The major factors associated with graft failure were transplants from male donors and the age of the patient (or donor). Among male recipients of male donor-derived grafts a low dose per kilogram of nucleated cells, progenitor cells (colony forming unit-GM) and T cells was also associated with graft failure. Additional irradiation to 1,500 cGy, high dose corticosteroids posttransplant, and additional peripheral blood donor T cells did not decrease the incidence of graft failure. In addition, type of leukemia, time from diagnosis to transplant, an intact spleen, or the presence of antidonor leukocyte antibodies did not correlate with graft failure. To ensure engraftment of secondary transplants, further immunosuppression was necessary but was poorly tolerated. However, engraftment and survival could be achieved with an immunosuppressive regimen in which antithymocyte globulin and high dose methylprednisolone were administered both before and after infusions of secondary partially T- cell-depleted marrow grafts.