Primary mucinous tumors of the female genital tract have morphologic features similar to primary gastrointestinal adenocarcinomas, and distinguishing these malignancies may be extremely difficult. The purpose of this study was to characterize the immunostaining patterns of tumors of the female genital tract that show mucinous differentiation using cytokeratin 7 (CK7), CK20, and CDX2 and to evaluate the usefulness of these markers in differentiating these tumors from gastrointestinal tract adenocarcinomas and also from each other. A total of 64 cases were collected, including adenocarcinomas of the ovary (n=13), endocervix (n=16), endometrium (n=34), and vagina (n=1), all of which showed predominant mucinous differentiation. Intestinal mucinous differentiation was present in 11 of the cases (6 endocervical, 4 ovarian, and 1 vaginal adenocarcinoma). All tumors were at least focally positive for CK7 with the exception of 3 cases. The majority of tumors were negative for CK20 and CDX2. However, 25% of endocervical, 24% of ovarian, and 3% of endometrial adenocarcinomas were positive for CDX2, CK20, or both. The positivity for CDX2 and CK20 correlated with intestinal differentiation: 73% of all intestinal mucinous adenocarcinomas and 4% of all Müllerian mucinous adenocarcinomas showed positivity for the hindgut markers. In 70% of the tumors positive for CK20/CDX2, the intensity of CK7 stain was stronger than the intensity of either CK20 or CDX2 stain. In conclusion, immunostaining for CK7/CK20/CDX2 is helpful in distinguishing Müllerian subtype of mucinous gynecologic tumors from lower gastrointestinal tract malignancies. In gynecologic mucinous tumors with intestinal differentiation, the overlap of staining positivity may be a limiting factor. However, a dominant CK7 staining pattern was observed.