Early studies of murine allogeneic bone marrow transplantation have clearly shown that the removal of T cells from donor grafts prevents graft-versus-host disease (1-3). These experiments have led to numerous clinical trials involving T cell depletion (TCD) for GVHD prophylaxis (4-6). When donor marrow has been depleted of T cells, the incidence of GVHD in patients has decreased. However, other transplantrelated complications, especially decreased engraftment and increased leukemic relapse, have been reported. This review discusses the effects of TCD on engraftment and the ability of murine models to determine the factors that promote or inhibit engraftment. Although current clinical findings support a cautious attitude toward clinical TCD, the long-term benefits may outweigh the disadvantages.