Our study is to determine the presence of endometrial intraepithelial neoplasia (EIN) in endometrial biopsy specimens classified by the 1994 World Health Organization (WHO) criteria for endometrial hyperplasia. Endometrial biopsy specimens that were stained with hematoxylin and eosin (HE) were examined and categorized by the WHO 1994 criteria and for the presence of EIN as defined by the International Endometrial Collaborative Group. ${eta}$-catenin expression was examined by immunohistochemistry. A total of 474 cases of HE stained endometrial biopsy tissues were reviewed. There were 379 cases of simple endometrial hyperplasia, 16 with simple atypical endometrial hyperplasia, 48 with complex endometrial hyperplasia, and 31 with complex atypical endometrial hyperplasia. Among the 474 endometrial hyperplasia cases, there were 46 (9.7%) that were classified as EIN. Of these 46 cases, 11(2.9%) were classified as simple endometrial hyperplasia, 1 (6.3%) as simple atypical endometrial hyperplasia, 6 (12.5%) as complex endometrial hyperplasia, and 28 (90.3%) as complex atypical endometrial hyperplasia. EIN was associated with a higher rate of ${eta}$-catenin positivity than endometrium classified as benign hyperplasia (72% vs. 22.5%, respectively, P<0.001), but a lower rate than endometrial adenocarcinoma (72% vs. 96.2%, respectively, P<0.001). In benign endometrial hyperplasia, high ${eta}$-catenin expression was noted in the cell membranes, whereas in EIN and endometrial adenocarcinoma high expression was noted in the cytoplasm. In conclusion, EIN is more accurate than the WHO classification for the diagnosis of precancerous lesions of the endometrium.