Context. - Dysregulation of mediators of apoptosis is associated with carcinogenesis. For biliary duct cancers, p53 gene mutation is an important contributor to carcinogenesis. Mutations in the p53 gene affect transcription of the Fas gene, resulting in lack of Fas expression on cell membrane. It has been previously shown that cloned Fas-negative but not Fas-positive human cholangiocarcinoma cells are resistant to anti-Fas-mediated apoptosis and develop tumors in nude mice. In addition, interferon gamma induces Fas expression in Fas-negative cholangiocarcinoma cells and makes them susceptible to apoptosis. Therefore, it becomes important to characterize immunophenotypic expression of p53 and Fas in normal and neoplastic human tissues of the biliary tract to further understand the pathogeoesis of the disease. To date, human studies to characterize differences in immunophenotypic expression of the Fas protein between intrahepatic and extrahepatic biliary duct cancers and in their precursor lesions have not been performed. Objective.-To report the immunophenotypic expression of p53 and Fas expression in various stages in the development of bile duct cancers (intrahepatic and extrahepatic tumor location) and their association with tumor differentiation. Design. - Thirty bile duct cancer samples (13 intrahepatic and 17 extrahepatic) from 18 men and 12 women who ranged in age from 44 to 77 years (mean age, 65.6 years) were retrieved from the surgical pathology files. Hematoxylin-eosin-stained slides were evaluated for the type and grade of tumor and dysplastic changes in the biliary tract epithelium. Additional slides were immunohistochemically stained with p53 and anti-Fas mouse monoclonal antibody. The pattern of Fas distribution and percentage of cells positive for p53 and Fas expression were determined. Results.-The percentage of Fas-expressing cells is significantly (P = .01) more frequently noted in extrahepatic tumors compared with intrahepatic tumors. Furthermore, Fas expression decreased from dysplastic epithelium to cholangiocarcinoma (P = .01), and this decreasing trend continued from well to poorly differentiated tumors. Nuclear p53 expression was not identified in normal and dysplastic epithelium but was noted in 30% of carcinomas (P = .02). Conclusion.-Fas expression is an early event in pathogenesis of bile duct cancers. Immunophenotypic expression of Fas is associated with well to moderately differentiated tumors but not with poor tumor differentiation.