The abnormal expression of blood group related antigens has been reported in many malignant tumours; however, such expression in cholangiocarcinoma has not been examined systematically. The expression of blood group-related antigens (A, B, H, Lewis(a), Lewis(b), Lewis(x), Lewis(y), carbohydrate antigen 19-9 and carcinoembryonic antigen) was investigated immunohistochemically in 75 cases of cholangiocarcinoma (31 peripheral type and 44 hilar type). In non-neoplastic bile ducts, A, B, and H antigens were expressed in large bile ducts, while Lewis(a,b,y) and carbohydrate antigen 19-9 were variably expressed in both large and small bile ducts. Lewis(x) and carcinoembryonic antigen was not found in non-neoplastic bile ducts. In cholangiocarcinomas, A, B, and H, antigens were more frequent in the hilar type than in the peripheral type, although the difference was not significant. The expression of the blood-group related antigens, particularly A, Lewis(a,b,y), carcinoembryonic antigen, and carbohydrate antigen 19-9, was frequent in the tumour cells in well differentiated adenocarcinomas, while their immunoreactivity was less frequent in poorly differentiated adenocarcinomas. The superanuclear and luminal expression of these antigens in carcinoma cells was frequent in well differentiated adenocarcinomas, and the diffuse, cell membranous and stromal expression of these antigens was relatively frequent in poorly differentiated adenocarcinomas and adenosquamous carcinoma. The A, B, and H immunoreactivity of both non-neoplastic bile ducts and cholangiocarcinomas was consistent with the host blood group type. These findings suggest that both the expression and intracellular distribution of blood group-related antigens in cholangiocarcinoma are related to the differentiation of cholangiocarcinoma and, possibly, to the parent structure.