Thirty-four cases of Sertoli-Leydig cell tumor were studied. All tumors were limited to the ovary at the time of initial surgery. Eight tumors were well differentiated, 15 were of intermediate differentiation, and 11 were poorly differentiated. Six cases contained heterologous elements. The less differentiated tumors occurred in patients with a lower median age and were more likely to produce androgenic manifestations. Follow-up of one year or longer was obtained in 15 patients, with an average follow-up in these patients of 6.1 years. Only one patient, who had a poorly differentiated tumor, died of the neoplasm in this series. Although follow-up was limited in this study, our findings suggest that the better differentiated tumors have a relatively favorable prognosis. This neoplasm is composed of sex-cord and stromal elements, and its components have the capacity to a greater or lesser extent to recapitulate the cells of the testis at different stages of development.