The origin and quantity of estrogen and androgen were measured in a postmenopausal woman with clinical signs of estrogen excess and a nonendocrine tumor of the ovary. The plasma androstenedione production rate was elevated 5-fold. The estrone production rate was also five times that normally expected for a postmenopausal women and could be accounted for totally by the extraglandular conversion of plasma and androstenedione. Following removal of the tumor, the concentration of plasma androstenedione and the estrone production rate fell dramatically to normal postmenopausal levels. It is concluded that this markedly increased androstenedione production was the result of excessive secretion of androstenedione by the hyperplastic stromal cells of the ovary containing the mucinous cystadenocarcinoma. The excessive prehormone production together with its normal extraglandular conversion to estrone resulted in the massive increase in endogenous estrogen formation.