Ovarian endometrioid carcinomas resembling sex cord-stromal tumors (ECSCSs) may simulate Sertoli cell tumors, Sertoli-Leydig cell tumors (SLCTs), and adult granulosa cell tumors (AGCTs), both clinically and pathologically. Differing clinical features and histologic findings are almost always successful in distinguishing these tumor types, although in some cases the differential diagnosis is difficult. Immunohistochemical staining of 17 ECSCSs, 14 Sertoli cell tumors or SLCTs, and 15 AGCTs was performed with the use of antibodies against cytokeratins (AE1/AE3, 902, and CAM 5.2), epithelial tumor-associated antigens (EMA, OM-1, B72.3, and carcinoembryonic antigen B1.1), vimentin, S-100, neuron-specific enolase, and lysozyme to determine the immunohistochemical profile of each tumor type and to define further the nature of the sex cord-like components in ECSCSs. All 17 ECSCSs, none of the 15 AGCTs, and one of 14 Sertoli cell tumors or SLCTs stained with EMA. Staining for OM-1 was almost as helpful diagnostically, with positive results for 15 of 17 ECSCSs, 0/15 AGCTs, and 1/14 Sertoli cell or SLCTs. Antikeratins were immunoreactive with all the ECSCSs as well as some of the AGCTs and Sertoli cell tumors or SLCTs. The B72.3 and B1.1 were immunoreactive with some ECSCSs and Sertoli cell tumors, but were nonreactive with AGCTs. Neuron-specific enolase was demonstrated in 11 of 17 ECSCSs, two of 14 Sertoli cell tumors or SLCTs, and 0 of 15 AGCTs. Vimentin, S-100, and lysozyme were least helpful in the differential diagnosis. These studies suggest that an immunohistochemical approach may be useful in the differentiation of ECSCSs and sex cord-stromal tumors. Furthermore, it supports the conclusion that the sex cord-like cells in ECSCSs are not Sertoli or granulosa cells, but cells of surface epithelial type growing in architectural patterns similar to those of sex cord-stromal tumors.