Abstract: Objective To evaluate the effect of tamoxifen(TAM)on endometrium in postoperative breast cancer patients treated with TAM. Methods Hysteroscopy and endometrial biopsies were performed in 69 patients having abnormal vaginal bleeding or abnormal thickness of endometrial by ultrasound in TAM group and in 60 patients with postmenopausal bleeding without tamoxifen as control group.TAM group was divided into premenopausal and menopausal group or three groups including T≤1 year,1 year＜T＜2 years and T≥2 years according to the time with tamoxifen. Results The frequency of endometrial polyps and hyperplasia was higher in TAM group,especially in menopausal group.All the atypical hyperplasia and endometrioid carcinoma cases occurred in menopausal group.The occurrence of endometrial disease increased along with the duration of taking TAM. Conclusion Risk of endometrial disease in postoperative breast cancer patients treated with TAM will be increased,especially in menopausal patients taking TAM more than 2 years.