Ovarian Sertoli cell tumors are rare, and their morphologic spectrum, behavior, and factors influencing the latter are not clearly established. They may be mimicked by many different tumors, some of them more frequent than Sertoli cell tumors; immunohistochemistry may aid in this differential, but its role has not been analyzed in a large series. We studied the clinicopathologic features of 54 Sertoli cell tumors, including the immunohistochemical profile of 23 of them. The patients, 6 of whom had Peutz-Jeghers syndrome, ranged from 2 to 76 years of age (mean, 30 years). Eleven patients had estrogenic and 4 had androgenic manifestations. The tumors ranged from 0.8 to 30 cm, with the majority being in the range of 4 to 12 cm. They were all unilateral, usually solid, and often yellow. The predominant microscopic pattern was tubular, seen, albeit often only focally, in all tumors; other patterns were cords or trabeculae (28), diffuse (21), pseudopapillary (4), retiform (3), islands or alveolar arrangements (3), and spindled (3). The tubules were solid or hollow with the former being somewhat more common. Delicate septa were occasionally seen and were conspicuous in areas of one tumor. The stroma was abundant in 15 tumors with marked sclerosis in 4. The cells usually had pale to occasionally densely eosinophilic cytoplasm, but 6 tumors were composed of cells with prominent foamy cytoplasm, falling in the category of "lipid-rich" Sertoli cell tumor, and one had cells with clear non-foamy cytoplasm. Forty-four tumors were stage I (42 of them were stage Ia and 2 were stage Ic), 1 was stage II, 3 were stage III, and 6 were not adequately staged. Follow-up was available for 27 patients with stage I tumors, and all were alive and well at last follow-up except for 2 patients with stage Ia and 1 with stage Ic disease. Those 3 patients had pelvic-abdominal recurrences 18, 36, and 9 months, respectively, after the initial diagnosis. Two of the three clinically malignant stage I tumors had moderate to severe cytologic atypia and brisk mitotic activity (>5 or more mitoses/10 high power fields [HPFs]), and one of these had tumor cell necrosis. Among the 10 clinically benign stage I tumors with more than 5 years of follow-up, only 3 had >5 mitoses/10 HPFs, but none had more than mild cytologic atypia and none had tumor cell necrosis. Two of the three patients with stage III disease had follow-up information and one was alive at 16 months and the second developed splenic metastases 2 years after the initial diagnosis. Two of the three stage III tumors had at least moderate cytologic atypia and brisk mitotic activity. Immunohistochemical stains showed positivity for AE1/3-Cam5.2 in 15 of 23 tumors; Epithelial membrane antigen (EMA) was negative in all the tumors. Inhibin was positive in 18 of 22 tumors, calretinin in 10 of 20, CD99 in 19 of 22, vimentin in 17 of 18, smooth muscle actin in 4 of 18, neuron specific enolase in 8 of 16, S-100 in 2 of 20, and chromogranin was negative in all 21 cases studied. Although Sertoli cell tumors usually have a distinctive tubular pattern that facilitates the diagnosis, other patterns may occasionally predominate, causing confusion with various other primary and metastatic ovarian tumors. EMA, inhibin, and chromogranin represent the most helpful triad of immunomarkers serving to exclude two common mimics of Sertoli cell tumors (endometrioid carcinoma [inhibin-; EMA+; chromogranin-] and carcinoid tumor [inhibin-; EMA+; chromogranin+]). Although CD99 and calretinin are often expressed in these tumors, they are much less specific and not as helpful in the differential diagnosis. Most Sertoli cell tumors are stage I, unilateral, cytologically bland, and clinically benign, but occasional examples are high stage, and about 11% of stage I tumors have worrisome histologic features that may portend an adverse outcome. The tumors typically occur in young females, sometimes children who typically present with sexual precocity, and occasional patients have Peutz-Jeghers syndrome.