In histopathologic diagnosis of the endometrium, it is very very difficult to distinguish atypical endometrial hyperplasia from well differentiated endometrioid adenocarcinoma. Some points of histopathologic diagnosis of the endometrium in HE stain and immunohistochemical staining are discussed. Marked proliferation of atypical epithelial cells, such as papillary, confluent or cribriform growth and desmoplasia of stroma are suggested presence of stromal invasion in HE stain. Immunohistochemical stain of p53, CD44 and cyclin A are negative for all reported cases of atypical hyperplasia and positive for some cases of endometrioid carcinoma, but, not all cases of endometrioid carcinoma are positive, so these immunohistochemical stains are not enough for diagnosis of endometrioid adenocarcinoma. In conclusion, stromal invasion in the HE stain is a most important point to diagnose endometrioid adenocarcinoma.