Undifferentiated carcinoma arising in the endometrium is considered a rare neoplasm with only a few studies published thus far. This limited number of studies is most likely a reflection of the underrecognition of this tumor because of a lack of diagnostic criteria to separate it from endometrial endometrioid adenocarcinoma, FIGO grade 3. In this study, we present the clinicopathologic features of 16 cases of endometrial undifferentiated carcinoma. In addition, we review the clinicopathologic features of 33 cases of endometrial endometrioid adenocarcinoma, FIGO grade 3, and compare them with the undifferentiated cases. The age of the 16 patients with undifferentiated carcinoma of the endometrium ranged from 40 and 69 years (mean, 59 years). Stage was known in 13 patients. Six (46%) patients presented with early stage disease (4 stage I and 2 stage II). Seven (54%) patients presented with advanced stage disease (2 stage III and 5 stage IV). Staging information was not available for 3 patients. Undifferentiated carcinoma was characterized by a proliferation of medium-sized, monotonous, epithelial cells growing in solid sheets with no specific pattern. Glands were not identified. Keratin immunostaining was focally positive in 11 of 12 cases, and EMA was focally positive in all 12 cases. The age of the 33 patients with endometrial endometrioid carcinoma, FIGO grade 3, ranged from 40 to 90 years (mean, 68 years). Twenty-three (70%) patients presented with early stage disease (21 stage I and 2 stage II), and 10 (30%) patients presented with advanced stage disease (8 stage III and 2 stage IV). Focal glandular differentiation was seen in all cases. The solid component was different from the one seen in the undifferentiated carcinomas because well demarcated trabeculae, cords, or groups of cells were identified in all cases. The tumor cells in the solid areas resembled the cells in the glandular component of the tumor. Immunoperoxidase studies for keratin and EMA were positive in 23 of 23 cases. Twelve of the 16 (75%) patients with undifferentiated carcinoma died of disease; 10 (62.5%) of them within 5 years after diagnosis. In contrast, 13 of 33 (39.4%) patients with endometrial endometrioid carcinoma, FIGO grade 3, died of disease. Twelve (36.4%) died within 5 years after diagnosis. In summary, undifferentiated carcinoma of the endometrium appears to be more aggressive than endometrial endometrioid adenocarcinoma, FIGO grade 3. Its proper recognition is important for prognosis and potentially for therapy.