A case of primary endodermal sinus tumor (EST) of the endometrium in a 28-year-old woman is described. EST has been reported to arise in several extragonadal sites, but to the authors' knowledge this is the first documented instance of origin in the endometrium. The histologic study is verified by the detection of alpha-fetoprotein in the tumor by an immunoperoxidase-peroxidase-antiperoxidase technique reported here. Displaced germinal cells, abnormal ovum, and residual fetal tissues are considered as possible origins of the neoplasia.