There is controversy regarding the prevalence of underlying endometrial adenocarcinoma among women with a diagnosis of atypical endometrial hyperplasia. This study further defines that risk. At our institution atypical endometrial proliferations non-diagnostic for invasive adenocarcinoma are diagnosed as either atypical endometrial hyperplasia (ATHY) or as an "atypical proliferative lesion of the endometrium, suggestive but not diagnostic of atypical endometrial hyperplasia" (APL). Between 1996 and 2003, these diagnoses were made on either endometrial biopsy or endometrial curettings in 60 women who subsequently received a hysterectomy. Endometrial adenocarcinoma was identified in 48% (29/60) of the hysterectomy specimens. Age and sampling method had no significant impact on the prevalence of adenocarcinoma. Adenocarcinoma was no more likely to be subsequently identified when a woman had a preoperative diagnosis of ATHY (24 of 52, 46%) compared to APL (5 of 8, 63%). In some women with a diagnosis of ATHY a comment was made in the report that "carcinoma cannot be ruled out". These cases had a significantly higher prevalence of underlying adenocarcinoma (16 of 25, 64%) compared to cases of ATHY in which such a comment was not made (8 of 27, 30%) (p = 0.025). In conclusion, there is a high prevalence of underlying endometrial adenocarcinoma among women undergoing hysterectomy for any of atypical endometrial proliferation.