In order to distinguish endocervical adenocarcinoma from endometrial adenocarcinoma, an immunoperoxidase stain for carcinoembryonic antigen (CEA) was tried. All of 10 endocervical adenocarcinomas revealed CEA and an adenosquamous carcinoma in the uterine cervix also showed it, while a mesonephroid adenocarcinoma in the uterine cervix did not. The immunohistochemical reaction products for the antigen were not observed in the glandular structures of 20 endometrial adenocarcinomas, although CEA was detected in all foci of squamous epithelial metaplasia occurred within 7 endometrial adenocarcinomas. CEA was detected in the endocervical type of glandular epithelium within a special endometrial adenocarcinoma containing predominantly endocervical type glandular epithelium. The immunoperoxidase staining pattern for CEA in the endocervical adenocarcinoma was related to the degree of histological differentiation of tumors, as follows; in the well differentiated glandular structure CEA was located on the luminal surface, while it was detected in the whole cytoplasm of tumor cells within the moderately and poorly differentiated areas. In conclusion, the immunoperoxidase stain for CEA would be useful for estimating malignancy of glandular structures within the uterus, distinguishing endocervical adenocarcinoma from endometrial adenocarcinoma, and grading of histological differentiation of endocervical adenocarcinoma.