Fas ligand (FasL) and perforin pathways not only are the major mechanisms of T cell–mediated cytotoxicity but also are involved in homeostatic regulation of these T cells. In the present study, we tested whether CD8⁺ donor T cells that are deficient in both perforin and FasL (cytotoxic double deficient [cdd]) could induce graft-versus-host disease (GVHD) in a major histocompatibility complex class I–mismatched lethally irradiated murine model. Interestingly, recipients of cdd CD8⁺ T cells demonstrated significantly greater serum levels of interferon gamma and tumor necrosis factor alpha and histopathologic damage from GVHD than wild-type (wt) T cells on day 30 after allogeneic bone marrow transplantation (P < .05). Wt and either perforin-deficient or FasL-deficient CD8⁺ T cells expanded early after transplantation followed by a contraction phase in which the majority of expanded CD8⁺ T cells were eliminated. In contrast, cdd CD8⁺ T cells exhibited prolonged expansion and reduced apoptosis to alloantigen stimulation in vivo and in vitro. Together these results suggest that donor cdd CD8⁺ T cells expand continuously and cause lethal GVHD, and that both perforin and FasL are required for the contraction of allo-reactive CD8⁺ T cells.