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Undifferentiated endometrial carcinoma (UEC) is a rare and poorly recognized entity, associated with a poor outcome. The clinical, pathologic, and immunohistochemical features of 17 cases diagnosed at our center are described. The median age was 60 yr. Postmenopausal bleeding was the most common presenting symptom (76.9%). Most patients presented with advanced stage (64.7%). Total hysterectomy with bilateral salpingo-oophorectomy was the commonly offered surgical treatment (80.0%). Nine (52.9%) patients received adjuvant treatment. The median overall survival was 11 mo. Pure UEC was seen in 8 cases (47.0%), while dedifferentiated carcinoma in 5 cases (29.4%). The epithelial component was part of carcinosarcoma or was mixed with serous carcinoma in 2 cases (11.8%) each. Positivity for one or more of the cytokeratin cocktails, mostly as strong focal staining, was evident in 16 cases (94.1%). PAX-8 was negative in 13 cases (86.7%). BRG-1/SMARCA4 was lost in 3 cases (20.0%). Eleven cases (64.7%) were MLH1/PMS2 deficient. Ten cases (66.7%) were positive for programmed death ligand 1, with positivity in 10%, 20%, and 100% of tumor cells detected in a single case each. Only 2 of 11 (18.2%) referral cases were correctly diagnosed as UEC. UEC is a rare tumor that is frequently misdiagnosed. A panel of immunostains is necessary to make the correct diagnosis. The range of positivity for programmed death ligand 1 testing suggests that immunotherapy might be considered in the adjuvant setting, especially with the poor response of this tumor to traditional therapies.