Ovarian venous concentrations of testosterone (18.7 ng/mL and 8.2 ng/mL) were three to six times higher than the peripheral concentrations (2.8 ng/mL) in a hirsute postmenopausal woman. She had cystic hyperplasia of the endometrium and atypical hyperthecosis of the ovary. Postoperatively, the testosterone levels returned to normal. From the results of the immunoperoxidase reaction, the luteinized stromal cells of the ovaries were the site of increased production of the testosterone and estradiol. By the same technic, these cells were negative for LH but strongly positive for FSH. From these data, the authors conclude that the luteinized cells were the primary source of the excessive testosterone, that the same cells were the direct and indirect (by peripheral testosterone conversion) source of estradiol, that LH was very likely not involved in the process of steroid hormone production, and that FSH may be the trophic stimulus responsible for the functional activity of the luteinized stromal cells in this virilized postmenopausal woman.