OBJECTIVE: The most commonly used classification system for endometrial hyperplasia is the World Health Organization system which is based on subjective criteria. Another classification system is endometrial intraepithelial neoplasia (EIN) system which uses diagnostic criteria including cytological demarcation, crowded gland architecture, minimum size of 1 mm, and careful exclusion of mimics, and aims to identify a precancer or cancer. The objective of this study was to compare the two classification systems in terms of predicting the presence of a coexistent cancer in surgically treated patients. METHODS: Biopsy and hysterectomy specimens of 49 women who were subjected to surgery with a preoperative diagnosis of endometrial hyperplasia (EH) according to the WHO system were re-evaluated retrospectively by using EIN system. RESULTS: Among the 49 patients, 69.4% had complex atypical EH and 75.5% had EIN at biopsy specimens. EIN was detected in 94.1% of complex atypical EH, and 41.7% of non-atypical EH. Nine women (18.4%) had endometrial cancer. Among women with cancer, all had complex atypical EH or EIN. The rate of coexistent endometrial cancer was 26.5% in women with complex atypical EH and 24.3% in women with EIN. CONCLUSION: Diagnoses of atypical or complex atypical EH and EIN had similar sensitivities and negative predictive values in predicting the coexistent endometrial cancer. Either of these two classification systems may be used safely when an experienced pathologist is available. However, use of the objective EIN system may be preferred whenever possible to prevent diagnostic errors in centers where an experienced pathologist is not available.