Objective To study the expressions of ER and 17β-HSD₁ in endometrial polyps, and to clarify the pathogenesis of endometrial polyps in molecular level. Methods Endometrial polyps from 47 patients were taken as the study group; at the same time, normal endometrial tissue far from endometrial polyps were taken from the same 47 patients as the control group 1 and normal endometria from 50 subjects were taken as the control group 2 who underwent hysteroscopy. Immunohistochemical examination was conducted to detect ER and 17β-HSD₁ expressions which were compared between the groups. Results The positive expression rate of ER in polyps group was 80.9%, which was 76.6% for 17β-HSD₁. The rates were higher than those of the control group(P＜0.01). The expressions of ER and 17β-HSD₁ were related to the risk of endometrial polyps(r=0.619, P＜0.01). Conclusions Excessive expressions of ER and 17β-HSD₁ in premenopausal endometrial polyps may be the cause of occurrence and development of endometrial polyps.