Regenerating islet-derived family, member 4 (Reg IV) has been shown to be associated with colorectal carcinogenesis and gastric carcinogenesis through intestinal metaplasia. In this study, we examined Reg IV expression in the gallbladder and gallbladder carcinoma, and measured Reg IV levels in sera from patients with gallbladder carcinoma. Quantitative reverse transcription-polymerase chain reaction revealed that high Reg IV levels were identified in 17 of 31 gallbladder carcinomas, whereas there was no apparent amplification in normal gallbladders. Immunohistochemically, although only a small part of the epithelium with intestinal metaplasia in 2 of 4 cases with adenomyomatosis showed Reg IV expression, Reg IV was negative in all cases with normal gallbladder (n = 15) and cholelithiasis (n = 13). In contrast, 34 (56%) of 61 gallbladder carcinomas were positive. Expression was more frequently observed in well to moderately differentiated than in poorly differentiated adenocarcinomas and significantly correlated with expression of caudal-related homeobox transcription factor (a candidate for involvement in the induction of intestinal metaplasia). Multivariate analysis revealed negative Reg IV expression, as well as hepatic parenchymal invasion, to be independently associated with a poor prognosis in patients with advanced gallbladder carcinoma. Before surgical resection, 4 (33%) of 12 patients with gallbladder carcinoma had high serum Reg IV levels, whereas Reg IV was never elevated in 12 patients with benign diseases. The serum levels of Reg IV decreased after surgical resection of the tumors. These results suggest that Reg IV is involved in gallbladder carcinoma carcinogenesis through intestinal metaplasia and is associated with relatively favorable prognosis in patients after surgery. The serum level of Reg IV may be of use or indicative of neoplasia.