Immunohistochemistry can be an important part of the diagnosis of Sertoli cell tumor of the ovary, including distinction from non-sex cord-stromal tumors such as the sertoliform variant of endometrioid carcinoma and carcinoid. Several good markers for this differential diagnosis have been identified, particularly inhibin, Wilms tumor 1 gene product (WT1), epithelial membrane antigen, and chromogranin; however, many available markers have limitations to some degree. Steroidogenic factor 1 (SF-1; adrenal 4-binding protein; Ad4BP) is a nuclear transcription factor involved in gonadal and adrenal development. In the testes, SF-1 is expressed in Sertoli cells. Immunohistochemical expression of this marker in ovarian sex cord-stromal tumors, including utility for differential diagnosis, has not been rigorously evaluated. As an extension of our previous immunohistochemical studies of ovarian Sertoli cell tumor, expression of SF-1 and comparison with WT1 and inhibin were assessed in 111 primary ovarian tumors: 27 Sertoli cell tumors, 60 endometrioid tumors (including borderline tumors, conventional well-differentiated carcinomas, and sertoliform variants of carcinoma), and 24 carcinoids. SF-1 was expressed in 100% of Sertoli cell tumors but not in endometrioid tumors or carcinoid. WT1 was expressed in 100% of Sertoli cell tumors and 17% of endometrioid tumors; all carcinoids were negative. Inhibin was expressed in 96% of Sertoli cell tumors and 2% of endometrioid tumors (4% of conventional well-differentiated carcinomas); all carcinoids were negative. The extent of expression of all 3 markers was similar in Sertoli cell tumor but greatest for WT1: 63%, 96%, and 78% of cases showed expression of SF-1, WT1, and inhibin, respectively, in more than 50% of tumor cells. Immunohistochemical composite scores combining both extent and intensity of staining in positive cases were calculated for Sertoli cell tumor (possible range: 1-12). Combined extent/intensity of immunostaining was similar for all 3 markers, but WT1 showed the most robust immunoreactivity in positive cases (mean immunohistochemical composite scores for SF-1, WT1, and inhibin: 6.1, 10.8, and 7.8, respectively). We conclude that for the differential diagnosis with endometrioid tumors and carcinoid of the ovary, SF-1 is a sensitive and specific immunohistochemical marker for Sertoli cell tumor and that SF-1 is diagnostically comparable with other good sex cord-stromal markers.