Müllerian inhibiting substance (MIS), alpha-inhibin, and CD99 are expressed by normal and neoplastic sex cord and stromal gonadal derivatives. The expression of MIS, inhibin, and CD99 was evaluated by immunohistochemistry in a series of 19 sex cord-stromal tumors and seven ovarian endometrioid carcinomas resembling sex cord-stromal tumors. Normal ovaries and testes from patients with the androgen insensitivity syndrome were used as control. All three markers were detected in primordial ovarian follicles and immature seminiferous tubules. Both inhibin and MIS were expressed by all sex cord-stromal tumors, but inhibin immunostaining appeared stronger and more diffusely distributed. CD99 was expressed by all adult and juvenile granulosa cell tumors, but only focally by Sertoli-Leydig cell tumors. In Sertoli-Leydig cell tumors, the three markers were predominantly expressed by the Sertoli cells. None of the seven endometrioid carcinomas with a sex cord-stromal pattern was positive for MIS or inhibin; two of them reacted for CD99, but their staining patterns were not membranous. Inhibin immunoreactivity was also detected in foci of stromal luteinization, occasionally present in these tumors. MIS, inhibin, and CD99 are potentially useful markers in the differential diagnosis between sex cord-stromal tumors and endometrioid carcinomas resembling sex cord-stromal tumors.