Undifferentiated endometrial carcinoma (UEC) is a relatively uncommon neoplasm with only few studies published thus far. It has always been a diagnostic challenge because of the lack of proper definition cited in most of the standard textbooks. Recently however, a few studies have highlighted the clinicopathologic features of UEC. The distinctive morphology of UEC was noted by the group from MD Anderson Cancer Center, which enabled them to establish the defining criteria. It appears to be more aggressive than endometrial endometrioid adenocarcinoma, FIGO (International Federation of Gynecology and Obstetrics) grade 3, its main differential diagnosis. Proper recognition of this entity is important owing to its aggressive behavior.