To date, seven apomucins have been characterized and their expression in malignant and premalignant lesions is under evaluation. In this study, we examined the expression of MUC1, MUC2, MUC3, and MUC5/6 apomucins in cholangiocarcinoma (CC) and biliary epithelial dysplasia. We used 14 liver samples from patients with hepatolithiasis and CC, 11 with hepatolithiasis showing biliary epithelial dysplasia, 31 with CC alone (19 hilar, 10 peripheral, and 2 unclassifiable), and 14 with combined hepatocellular-cholangiocellular carcinoma (HC-CC) and immunohistochemically characterized the expression profiles of apomucins. Nondysplastic biliary epithelial cells in the intrahepatic large bile ducts constantly expressed MUC3 apomucin. MUC5/6 and MUC3 apomucin expression was widespread in dysplastic biliary epithelial cells in hepatolithiasis, although the latter was reduced or absent in dysplastic foci. CC extensively expressed MUC1 apomucin and focally expressed MUC2 apomucin. In addition, CC of the hilar type, including those with hepatolithiasis, frequently expressed MUC3 apomucin (68% and 57%, respectively), whereas those of the peripheral type infrequently expressed MUC3 apomucin (10%) (P < .01). MUC5/6 apomucin was more frequently expressed in well-differentiated (89%), compared with poorly differentiated CC (42%) (P < .01). Cholangiocellular elements of combined HC-CC frequently expressed MUC1 apomucin, although they rarely expressed MUC2 and MUC3 apomucin and infrequently expressed MUC5/6 apomucin. The frequent and aberrant expression of "gastric type" MUC5/6 apomucin in biliary epithelial dysplasia, as well as in CC, suggests that biliary epithelial cells gain a gastric apomucin phenotype during carcinogenesis. MUC3 apomucin expression in CC is a marker that suggests that CC arises in the intrahepatic large bile ducts.