Several studies have reported on the use of antibodies to monoclonal carcinoembryonic antigen (CEA) and vimentin (VIM) to distinguish between adenocarcinomas of endometrial (EM) and endocervical (EC) origin, with variably enthusiastic results. It is still unclear whether site of origin or pathway of differentiation (endometrioid [em] versus mucinous [m]) is more important in predicting immunohistochemical differences. In the present study, paraffin blocks from adenocarcinomas of known origin were retrieved and immunostained with monoclonal antibodies to VIM and CEA, as well as cytokeratins (CK) 4, 18, and 20, estrogen receptor (ER), and progesterone receptor (PR). Positivity was scored on a scale from 0 to 12, with emphasis on the pattern of differentiation (tumors with mixed patterns received separate scores for the em and m foci). Mean CEA scores for emEM (n = 27), mEM (17), mEC (10), and emEC (6) were 0.4, 0.9, 5.1, and 1.2, respectively. VIM scores were 6.9, 1.3, 0, 4.4; ER, 5.7, 4.2, 0, 1.6; PR, 7.6, 2.8, 0.1, 6.0; CK4, 9.2, 4.4, 8.5, 10.6; CK18, 6.4, 3.4, 5.5, 8.4; CK20, 0.7, 0, 0.5, 0.4. Both site and differentiation influenced these results, with the latter more important for VIM and PR, the former for ER, both for CEA (only mEC was frequently strongly positive), and neither for the CKs studied. No one stain or combination reliably distinguished endometrial from endocervical origin. The only immunostaining pattern that might identify a site of origin with more accuracy than hematoxylin & eosin evaluation alone is the combination of high VIM and ER scores in an endometrioid carcinoma, suggesting with about 95% accuracy in this series an endometrial origin of the tumor.