Ovarian endometrioid adenocarcinomas may have an extremely variable morphologic appearance and mimic a number of other epithelial malignancies as well as nonepithelial tumors. We report the clinicopathologic features of a small series of ovarian endometrioid adenocarcinomas (n=5), 4 of which were received in consultation, which were originally diagnosed as or were suspected to represent serous borderline tumor, with or without a component of low-grade serous adenocarcinoma. The patients were aged between 47 and 74 yr (mean, 60 yr), and all tumors were unilateral and Stage 1C. Serous borderline tumor was suspected on the basis of the predominant architectural pattern with prominent papillary formations consisting of rather bland epithelial cells covering stromal cores which projected into cystic spaces. In all cases, there were areas of typical endometrioid adenocarcinoma, although these foci were minor. Features useful in confirming an endometrioid neoplasm, not all of which were present in every case, were a monomorphous cell population, areas of cytoplasmic clearing, areas of more pronounced nuclear atypia, and mitotic activity than is typical of low-grade serous neoplasms, squamous elements, endometriosis and absent or only focal WT1 immunohistochemical staining. The close mimicry of a serous borderline tumor by an endometrioid adenocarcinoma is a diagnostic pitfall which has not been reported in the literature and represents yet another example of the propensity for ovarian endometrioid adenocarcinomas to mimic other neoplasms. Pathologists should consider an endometrioid adenocarcinoma when faced with a presumed serous borderline tumor with any of the features listed above. Extensive sampling may be of value in revealing more typical areas of endometrioid neoplasia and negative staining with WT1 of use in excluding a serous neoplasm.