A tumor of the right ovary in a 21-year-old single woman is reported. Secondary amenorrhea, hirsutism, acne and deepening of the voice were associated with the tumor. Light and electron microscopic examinations showed that the tumor was composed of cells resembling Sertoli and Leydig cells of the testis in their cytology features and growth patterns. High levels of circulating dehydroepiandrosterone, androstenedione, testosterone and alpha-fetoprotein (AFP) were found preoperatively. Preoperative estrogen and progesterone levels were all slightly above the upper limits of normal for females. These hormone and AFP levels fell to within the normal range after removal of the tumor. Direct hormone and AFP production of this tumor was confirmed by immunohistochemical techniques and long-term cell cultures in vitro. This is possibly the first report on a Sertoli-Leydig cell tumor in which AFP has been identified in the patient's plasma, in part of the tumor cells and the culture fluid.