OBJECTIVES: To examine the prevalence of undiagnosed endometrial carcinoma (EC) among women with a preoperative diagnosis of atypical endometrial hyperplasia (AEH) in correlation to age, BMI and menopause. METHODS: Data extracted from the Danish Gynecological Cancer Database (DGCD) covering women diagnosed with AEH between January 1, 2005 and November 1, 2010 undergoing surgery. DGCD is a multidisciplinary, nationwide, clinical database of all cases of gynecological cancer and AEH in Denmark diagnosed after January 1, 2005. Registration is mandatory. Primary outcome was preoperative- and postoperative diagnoses. Secondary outcomes were relationship to BMI, age and menopause. RESULTS: The preoperative diagnosis of AEH was retained in 41% of 773 cases and 59% had endometrial cancer. Of the cancer cases, 18% had more than Stage I disease and 3% were non-endometrioid. Cancer risk was significantly related to age (p