The presence of signet ring cells in a carcinoma within the uterine corpus strongly raises the possibility of a metastasis from a primary tumor in the breast, gastrointestinal tract, or elsewhere. Signet ring cells are extremely rare in primary endometrial adenocarcinomas with only a single prior case report. We report 2 cases of primary endometrial adenocarcinoma, one of mucinous and the other of endometrioid type, with a significant component of signet ring cells. One of the neoplasms arose within adenomyosis. In reporting these cases, we illustrate that the presence of signet ring cells does not preclude a primary endometrial adenocarcinoma.We discuss signet ring cells in the endometrium.