Fas (APO-1/CD95) is a cell surface receptor that mediates apoptosis when it reacts with Fas ligand (FasL) or Fas antibody. In this study, we analyzed Fas and FasL expression in normal esophageal mucosa and esophageal squamous cell carcinomas. Reverse transcriptase-PCR revealed that Fas, soluble Fas, and FasL were expressed in all eight esophageal squamous carcinoma cell lines analyzed. Furthermore, it was demonstrated that FasL expressed in esophageal carcinoma cells is functional because coculture experiments using FasL-expressing TE-15 esophageal carcinoma cells resulted in apoptosis of Jurkat T leukemia cells, which are sensitive to Fas-mediated apoptosis. Immunohistochemistry of Fas and FasL showed that they are constitutively expressed in normal esophageal mucosa, FasL being predominantly in the basal and suprabasal layers, whereas Fas is in more differentiated layers, i.e., rows of polyhedral cells of the intermediate layers and squamous cells forming the outer layers. In 18 of 19 invasive esophageal squamous cell carcinomas, FasL expression was found in >50% of tumor cells. In contrast, most tumors (15 of 19, 79%) either showed no Fas expression or showed expression in <5% of tumor cells. These alterations were already detected in dysplasia and carcinoma in situ. These results suggest that up-regulation of FasL and down-regulation of Fas expression are early and frequent events associated with the evolution of esophageal squamous cell carcinomas.