Unopposed endogenous and exogenous estrogenic stimulation has been considered by most investigators to have a role in the pathogenesis of carcinoma of the endometrium. Although a few cases of "sarcomas" of the endometrium that had developed in an estrogenic setting have been reported, a clear-cut association between estrogenic stimulation and these forms of endometrial cancer has not been established. We report six cases of endometrial sarcomas complicating ovarian thecomas, polycystic ovarian disease, or prolonged estrogen therapy. Three ovarian thecomas, which are considered to be estrogenic tumors, were associated with endometrial malignant mullerian mixed tumor, mullerian adenosarcoma, and low-grade stromal sarcoma in postmenopausal women. Polycystic ovarian disease, a condition characterized by unopposed estrinism due to the peripheral conversion of excessive androstenedione to estrone, was found in a 27-year-old infertile woman with an endometrial malignant mullerian mixed tumor. A pure osteogenic sarcoma of endometrial stromal origin developed in a 28-year-old woman with gonadal dysgenesis (Turner's syndrome) who had received estrogens for 18 years. The sixth woman, with an empty sella turcica after radiation therapy of a pituitary adenoma, had an endometrial mullerian adenosarcoma at the age of 40 years after 16 years of estrogen therapy. None of these patients had had pelvic radiation therapy. The evidence from this series of cases and from six additional cases identified in the literature suggests that the risk of endometrial sarcomas may be increased by estrogen therapy or endogenous disorders that lead to unopposed estrogenic stimulation of the uterus.