The differential diagnosis of endocervical and endometrial adenocarcinomas can be improved by the demonstration of carcinoembryonic antigen (CEA) in tissue by means of immunoperoxidase staining. Tissue from 131 (80%) of 163 patients with endocervical adenocarcinoma but only 11 (8%) of 137 patients with endometrial adenocarcinoma was CEA-positive. The commonest exceptions were endocervical mesonephroid adenocarcinomas (which were CEA-negative) and endometrial adenosquamous carcinomas (which were CEA-positive). After exclusion of these on simple morphological criteria, 86 of 107 endocervical adenocarcinomas (80%) were CEA-positive, and all 122 endometrial adenocarcinomas were CEA-negative. The remarkable difference in the expression of CEA between endocervical and endometrial adenocarcinomas suggests a novel application of immunohistochemistry in routine clinical practice.