The choice of appropriate therapeutic plans for primary endocervical adenocarcinomas and endometrial adenocarcinomas depends on the site of origin of the tumor. The purpose of this study was to make clear whether the immunohistochemistry of the true cytokeratin 8/18 monoclonal antibody (Leica Microsystems, Newcastle, United Kingdom), instead of CAM 5.2 (Becton Dickinson Biosciences, San Jose, CA), has potential use in distinguishing between endocervical adenocarcinomas and endometrial adenocarcinomas. A tissue microarray was constructed using paraffin-embedded, formalin-fixed tissues from 34 hysterectomy specimens, including 14 endocervical adenocarcinomas and 20 endometrial adenocarcinomas. Using the Bond-Max autostainer (Leica Microsystems) and the associated Bond Refine Polymer Detection Kit, tissue array sections were immunostained with cytokeratin 8, 18, and 8/18 commercially available antibodies. The immunohistochemical expressions of all 3 markers, cytokeratin 8, 18, and 8/18 showed nonsignificant (P>0.05) frequency differences between the immunostaining results (positive vs. negative) in tumors of both gynecologic adenocarcinomas. Although CAM 5.2 has been reported to be helpful in distinguishing between primary endocervical adenocarcinomas and endometrial adenocarcinomas, we could not verify this point of view using the true cytokeratin 8/18 monoclonal antibody (Leica Microsystems). It has often been mistakenly cited that CAM 5.2 reacts with cytokeratin 8 and 18, and the results herein confer that there is a wrong impression that cytokeratin 8/18 is differentially expressed in these 2 gynecologic malignancies. In conclusion, the true cytokeratin 8/18 monoclonal antibody is of no use in distinguishing between primary endocervical adenocarcinomas and endometrial adenocarcinomas.