Mammaglobin (MGB), a secretory protein belonging to the uteroglobin/Clara cell protein family, is a sensitive marker for breast carcinoma, but is also reported to be expressed in the female genital tract and its neoplasms. Details of MGB expression pattern and its pathologic significance in the female genital tract have not been systematically studied. To investigate the potential use of MGB in gynecologic pathology practice, we tested MGB expression by immunohistochemistry on 47 endocervical adenocarcinomas (whole tissue sections of 13 invasive and 35 in situ) and 55 endometrial carcinomas (39 endometrioid and 16 nonendometrioid represented on a single tissue microarray). Nonneoplastic endocervical and endometrial tissues were also evaluated for MGB expression. MGB expression was detected in thirty (77%) of 39 of endometrioid endometrial adenocarcinomas compared with 4 (31%) of 13 endocervical adenocarcinomas. MGB was mostly negative in nonendometrioid endometrial carcinoma (negative in 14 [88%] of 16). Endocervical adenocarcinoma in situ (AIS) showed either weak (predominantly) or moderate (occasionally) expression in about 40% of the cases in comparison with strong positivity in benign endocervical glandular epithelium. Reduction of MGB staining was seen in transition from benign epithelium to AIS. These results confirm that MGB is not specific for breast carcinoma, but is also variably expressed in nonneoplastic and neoplastic endocervical and endometrial tissues. Frequent MGB expression in endometrioid endometrial adenocarcinoma is significantly different from nonendometrioid carcinoma. Hormone receptor status is not associated with MGB expression in endometrial carcinomas. Most endocervical adenocarcinomas are negative for MGB, in contrast to mostly positive endometrioid endometrial adenocarcinomas, however, MGB expression alone is not specific enough to distinguish these 2 tumor types. MGB expression is altered in neoplastic endocervical epithelium compared with normal, and may indicate its decreased expression in the process of early carcinogenesis. MGB may be a promising new adjunctive marker in gynecologic pathology.