Ovarian sex cord-stromal tumors are a morphologically diverse group of neoplasms that can mimic the appearance of other ovarian tumors. Because the treatment and prognosis of sex cord-stromal tumors differs substantially from those of other ovarian neoplasms, the development of an immunohistochemical panel to support the diagnosis of the former group of tumors would be useful. In this report, the utility of immunostaining for inhibin alpha, epithelial membrane antigen, MIC2 gene protein product, and keratin in the differential diagnosis of sex cord-stromal tumors was assessed in formalin-fixed, paraffin-embedded sections. In addition, the immunohistochemical staining pattern of ovarian small cell carcinomas (SCCs), hypercalcemic type, was analyzed in an attempt to clarify the histogenesis of these tumors. Thirty-two (97%) of 33 granulosa cell tumors (GCTs), 10 (91%) of 11 Sertoli-Leydig cell tumors (SLCTs), and 4 (8%) of 51 carcinomas showed inhibin alpha immunopositivity. None of the 3 lymphomas, 5 carcinoids, 6 dysgerminomas, or 12 SCCs showed inhibin alpha positivity. Eighteen (55%) GCTs, 6 (55%) SLCTs, 6 (12%) carcinomas, and 7 (58%) SCCs showed MIC2 gene expression. None of the GCTs and only one SLCT showed epithelial membrane antigen (EMA) positivity, although 92% of surface epithelial carcinomas and 75% of SCCs were immunoreactive. These data suggest that detection of inhibin immunoreactivity in an ovarian tumor that is EMA-negative provides both sensitive and specific support for the diagnosis of a sex cord-stromal tumor. Because SCCs generally stain for EMA but not for inhibin, it appears that SCCs probably represent a variant of surface epithelial tumor rather than a type of sex cord-stromal tumor.