OBJECTIVE To evaluate the prevalence of concurrent endometrial carcinoma in women diagnosed with atypical endometrial hyperplasia (AEH) by endometrial biopsy. METHODS We retrospectively analyzed the medical records of 126 patients who underwent hysterectomies for AEH diagnosed by endometrial biopsy from 1999 to 2008. AEH was initially diagnosed by dilatation and curettage (98 cases) or endometrial biopsy with a Z-sampler (24 cases). The remaining four cases were diagnosed by hysteroscopic polypectomy. The results of the endometrial biopsies were graded on an ordinal scale and were compared with pathologic features obtained at the hysterectomy. RESULTS In patients preoperatively diagnosed with AEH by biopsy, hysterectomy specimens revealed a rate of simple or complex endometrial hyperplasia without atypia of 27% with AEH and normal proliferative phases found in 54.7 and 7.9% of specimens, respectively. The incidence of endometrial carcinoma was considerably high (13/126, 10.3%). Eleven of 13 cases were confined to the endometrium and the remaining two were located at the adenomyosis without myometrial invasion. All patients with endometrial carcinoma displayed coexisting atypical complex hyperplasia following hysterectomy. CONCLUSIONS Biopsy specimens showing AEH, particularly atypical complex hyperplasia, are associated with a risk of coexisting endometrial carcinoma. When considering management strategies for women with a biopsy diagnosis of AEH, clinicians should take into account the considerable rate of concurrent endometrial cancer and the discrepancy with pathologic diagnosis. Treatment modalities may differ depending on population as the rates of concurrent endometrial cancer with AEH and myometrial invasion vary by geographical location.