The Reg I gene (regenerating gene) and its product (Reg protein) are a regenerating and/or proliferating factor(s) of pancreatic islet cells. The ectopic expression of REG Ialpha was shown in colorectal carcinomas, suggesting that REG Ialpha is related to their carcinogenesis. In this study, we examined the expression of REG I in intrahepatic cholangiocarcinoma (ICC) and its precursor lesion (biliary dysplasia). By polymerase chain reaction and in situ hybridization (ISH) studies using a total of 16 fresh liver specimens, REG Ialpha mRNA was demonstrated in 6 of 11 (55%) ICC cases, but in 0 of 5 (0%) normal livers. Immunohistochemistry for REG I protein was performed in 100 formalin-fixed, paraffin-embedded sections obtained from the 18 cases of ICC alone, 45 hepatolithiasis with ICC (n = 19) or biliary dysplasia (n = 26), 21 hepatolithiasis alone (all with hyperplasia), and 16 normal livers. In ICC, the expression of REG I protein was significantly dependent on the histologic differentiation; 12 of 13 (92%) cases in papillary and well-differentiated, 6 of 16 (38%) cases in moderately differentiated, and 0 of 8 (0%) cases in poorly differentiated types. Moreover, in the lesions of hyperplasia, low-grade dysplasia, and high-grade dysplasia in hepatolithiasis, REG I protein was expressed in 4 of 21 (19%), 7 of 12 (58%), and 13 of 14 (93%) cases, respectively. In normal liver, intrahepatic bile ducts were constantly negative for REG I protein. These findings suggest that neoexpression of REG I is a good marker for biliary mucosa at risk for development of ICC, and also that REG I plays a role in the early stages of biliary carcinogenesis, probably via a cell-proliferative effect.