Regenerating islet-deprived gene family, number 4 (REG4), is a novel marker for intestinal differentiation. We performed immunohistochemical studies on REG4, cytokeratin (CK)7, CK20, and caudal type homeobox 2 (CDX2) in 291 ovarian mucinous tumors. There were 226 primary tumors and 65 metastatic tumors. The primary tumors comprised 69/226 mucinous cystadenomas, 79/226 mucinous borderline tumors (64/79 intestinal-type and 15/79 endocervical-like tumors), and 78/226 mucinous carcinomas. We found that REG4 expression was significantly higher in mucinous borderline tumors (30/79, 38.0%) and primary mucinous carcinomas (26/78, 33.3%) than in mucinous cystadenomas (4/69, 5.8%; P<0.05). However, REG4 expression was more commonly associated with intestinal-type, borderline, mucinous tumors rather than the endocervical-like type (30/64 vs. 0/15, P<0.001). There was a significant correlation between the REG4 and CDX2 expression profiles in primary ovarian mucinous tumors (r=0.772, P<0.001). REG4, CDX2, and diffuse CK20 had higher expression frequencies in metastatic lower gastrointestinal adenocarcinoma than in primary mucinous tumors (P<0.01). The CK7/REG4 coordinate expression profile was comparable in diagnostic value to CK7/CK20 or CK7/CDX2 profile. We conclude that REG4 expression is common in mucinous borderline tumors of the intestinal type as it is absent in the endocervical-like form in this series. Expression of CK7/REG4 may contribute to the differential diagnosis between primary and metastatic ovarian mucinous tumors.