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Nongestational, nongonadal, and nonteratoma-related choriocarcinoma is an extremely rare entity. The tumor may be pure or associated with carcinoma. Six cases of primary endometrial carcinomas with the presence of syncytiotrophoblastlike beta-human chorionic gonadotropin (beta-HCG)-positive cells have been reported. We describe a case of endometrial carcinoma in a postmenopausal woman, that was composed almost entirely of syncytiotrophoblastic and cytotrophoblastic elements, except in the areas of lymphatic and myometrial invasion, where the neoplasm expressed a papillary serous carcinoma conformation with numerous psammoma bodies. There was not a clear transition between choriocarcinoma and serous carcinoma components. Immunohistochemically, beta-HCG was strongly positive in trophoblastic cells, whereas papillary areas expressed epithelial membrane antigen. These results suggest a double differentiation from malignant stem cells toward an extraembryonal germinal line and a papillary epithelial line. A poor prognosis is expected for this type of neoplasm with extraembryonic differentiation. In the present case, the histologic subtype of uterine papillary serous carcinoma could be an additional risk factor.