A 32-year-old woman, 3, para 2 with a testosterone producing tumor of the left ovary was studied endocrinologically. She complained of amenorrhea, hirsutism and abdominal mass. The peripheral testosterone level was 8.4 ng/ml, remarkably elevated. After the combined dexamethasone suppression/hCG stimulation test, the plasma testosterone level rose from 9 ng/ml to 15.1 ng/ml. Prior to removal of the tumor, venous samples were drawn directly from the right and left ovarian veins during surgery and simple total hysterectomy with bilateral salpingo-oophorectomy was performed. Testosterone levels were 40.5 ng/ml on the tumor side and 7.1 ng/ml on the normal side. After tumor removal, the plasma testosterone level fell from 8.4 ng/ml to less than 1.0 ng/ml within 24 hours. Histological examination of the left ovarian tumor revealed a Sertoli-Leydig cell tumor. In incubation of small specimens of tumor tissues in oxygenated Krebs bicarbonate buffer, the release of testosterone into the medium containing hCG was twice as high as that into the medium without hCG. These results of in vitro and in vivo studies suggest that this tumor was an hCG-dependent testosterone producing Sertoli-Leydig cell tumor.