Here, we present a rare case of intestinal type adenocarcinoma arising in mature cystic teratoma (MCT) and review all previously reported similar cases with emphasis on the immunohistochemical characteristics of prior cases. Nine prior cases of intestinal type adenocarcinoma arising in MCT have been previously reported. Two of the prior cases as well as this case have been associated with CA19-9 elevation. CK20 is consistently strongly positive and CK7 is usually negative in cases of intestinal type adenocarcinoma arising in MCT. This contrasts with mucinous tumors of the ovary which are known to be usually CK7 strongly positive and inconsistently CK20 positive. The pattern of strong CK20 staining and often negative or weak CK7 staining is common to mucinous neoplasms arising in MCT. These findings suggest that the less common subset of primary mucinous ovarian tumors that have strong CK20 staining and partial or negative CK7 staining may represent a group of tumors with germ cell origin rather than the more common surface epithelial origin.