The objective of our study was to examine the relationship between benign endometrial polyps and the underlying endometrium, analyzing factors associated with endometrial hyperplasia with and without atypia. Six hundred and ninety-four consecutive patients with benign endometrial polyps underwent hysteroscopic removal of the polyps combined with 2 biopsies of unremarkable endometrium. Hysteroscopic removal of endometrial polyps with 2 endometrial biopsies far from the base of the polyp were performed. The weight of each polyp was determined. Multivariable logistic regression analysis was used examining factors potentially associated with endometrial hyperplasia with and without atypia on endometrial biopsies. Overall, 18% of women had hyperplasia without atypia and 7.3% had atypia (with simple/complex hyperplasia) on hysteroscopically unremarkable endometrium. In postmenopause, 21.6% had hyperplasia without atypia, 12% atypia, and 1.2% adenocarcinoma on the sampled endometrium. At multivariable analysis, postmenopausal women with larger polyps had a 3.6-fold higher likelihood of atypia (odds ratio=3.6; 95% confidence interval: 1.3-10.3); in premenopause the likelihood of atypia was significantly associated with polyp weight and age above 40 years. Our findings suggest that important endometrial lesions are not always evident by hysteroscopic visualization. Therefore, endometrial biopsies should not be limited only to hysteroscopically evident lesions. Polypectomy should be combined with a histopathologic evaluation of the background endometrium, particularly in women with higher-risk characteristics.