I n vitro studies have demonstrated that intestinal intraepithelial lymphocytes (IEL) are constitutively cytotoxic; however, the mechanism and target of their cytotoxicity are unknown. Apoptosis of intestinal epithelial cells (IEC) and an increase in IEL numbers are classical signs of intestinal graft-versus-host disease (GVHD), although whether IEL can mediate IEC apoptosis directly in GVHD is unclear. Recent evidence suggests that target epithelial organ injury observed in GVHD is predominantly Fas-mediated; therefore, we investigated the possibility that IEL induce apoptosis of IEC through a Fas-mediated mechanism. Here, we demonstrate that the IEL isolated from normal mice readily display potent Fas ligand (FasL)-mediated killing activity after CD3 stimulation, and that IEC express Fas, suggesting that IEC are potential targets for FasL-mediated killing by IEL. In vitro, IEL isolated from GVHD mice have markedly increased FasL-mediated killing potential and are spontaneously cytolytic toward host-derived tumor cells predominantly through a Fas-mediated pathway. In vivo transfer of IEL isolated from GVHD mice induced significantly more IEC apoptosis in F1 wild-type mice than in Fas-defective F1lpr mice. Thus, these results demonstrate that FasL-mediated death of IEC by IEL is a major mechanism of IEC apoptosis seen in GVHD.