Background Various cancer cell lines express Fas ligand (FasL) and can kill lymphoid cells by Fas-mediated apoptosis in vitro. FasL expression was first demonstrated by us in colonic cancer and confirmed by others in extraintestinal human malignancies in vivo. Relative resistance of cancer cells to Fas-mediated apoptosis coupled with the ability to make a pre-emptive strike against infiltrating lymphocytes (by expressing FasL and triggering apoptosis in lymphocytes) theoretically confers a privileged status on a cancer and has been termed the Fas counterattack. The Aims of this study were to determine: (i) whether human esophageal carcinomas express FasL; and (ii) whether FasL expression is associated with increased apoptosis of tumorinfiltrating lymphocytes (TIL) in vivo, thereby contributing to the immune privilege of the tumor. Methods and Results Using in situ hybridization and immunohistochemistry respectively, FasL mRNA and protein were colocalized to neoplastic esophageal epithelial cells in all esophageal carcinomas (squamous, n=6; adenocarcinoma, n=2). Extent of FasL expression was variable, with both FasL-positive and -negative neoplastic regions occurring within individual tumors. TIL were detected by immunohistochemical staining for the leukocyte common antigen, CD45. FasL expression was associated with a mean four-fold depletion of TIL when compared with FasL-negative areas within the same tumors (range 1.6-12.0fold, n=6, p<0.05). Cell death of TIL was detected by dual staining of CD45 (immunohistochemistry) and DNA strand breaks (TUNEL, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling). There was a mean two-fold increase in detectable cell death among TIL in FasL-positive areas compared to FasL-negative areas (range 1.6-2.4-fold, n=6, p<0.05). In conclusion We demonstrate a statistically significant, quantitative reduction of TIL concomitant with significantly increased TIL apoptosis within FasL-expressing areas of esophageal tumors. Our findings suggest that Fas-mediated apoptotic depletion of TIL occurs in response to FasL expression by esophageal cancers, and provide the first direct, quantitative evidence to support the "Fas counterattack" as a mechanism of immune privilege in vivo in human cancer. Supported by a grant from the Health Research Board of Ireland.