We examined immunohistochemically the possible participation of the Fas/Fas ligand (FasL) system in intrahepatic cholangiocarcinoma (ICC) during the escape from immune surveillance, using 68 cases of ICC, 29 cases of normal intrahepatic large bile ducts, and 18 cases of biliary dysplasia. Apoptosis of tumor-infiltrating lymphocytes (TIL) was examined by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Fas was weakly expressed in normal intrahepatic bile ducts. Almost all biliary dysplasia and well-differentiated ICCs showed moderate to marked expression of Fas, while Fas expression was variable in moderately and poorly differentiated ICCs. Down-regulation of Fas expression was significantly correlated with histologic de-differentiation, vascular invasion, the size of ICCs, and short survival of ICC patients. By in situ hybridization, FasL mRNA were frequently and strongly expressed in biliary dysplasia compared with non-neoplastic intrahepatic bile duct. In well-differentiated ICCs, FasL mRNA expression was frequent and intense. But, the expression gradually decreased in moderately and poorly differentiated ICCs. Down-regulation of FasL mRNA expression in ICCs was correlated with perineural invasion and tumor size (over 4 cm) (P < .05). Apoptotic TIL were more frequent in ICC foci than in non-neoplastic foci remote from ICC foci. These findings suggest that a tumor evasion mechanism involving Fas/FasL exists in ICC; frequent and intense expression of FasL mRNA in well-differentiated ICCs enable them to escape immune surveillance by counterattacking Fas-bearing TIL. This counterattack becomes insensitive in poorly differentiated ICCs, in which the down-regulation of Fas gives them a resistance against the FasL-expressing TIL. These mechanisms may be involved in the tumor progression.