Atypical stromal cells of the lower female gynecologic tract have been specifically described in the vagina, vulva, and cervix, predominantly in the context of polyps. Rare cases of atypical stromal cells have been documented in the endometrium. We report a series of 15 examples of atypical stromal cells in the endometrium: 13 in endometrial polyps and two within endometrial stroma in curettage/biopsy specimens unassociated with polyps. The patients ranged in age from 45 to 82 years. Immunohistochemical studies were performed to aid in the identification of the origin of these atypical cells. The differential diagnoses included adenosarcoma, endometrial stromal sarcoma, and, less likely, malignant mesodermal mixed tumor (MMMT/carcinosarcoma). Similar to atypical stromal cells reported in other gynecologic sites, such cells discovered in the endometrium also appear to have a benign clinical course after complete excision or polypectomy (follow-up ranging from 1 month to 44 months). Accurate recognition of this lesion is essential to avoid unnecessary surgical overtreatment. Because of their rarity, limited available data, and lack of significant long-term follow-up, continued clinical monitoring of these patients would be prudent.