Objective: To investigate the effect of endometrial abnormalities in breast cancer patients treated with Tamoxifen (group I), Aromatase inhibitors (group II) or No treatment (group III). To determine the best approach for screening these patients for endometrial pathology. Methods: 117 patients at outpatient clinic, Departments of Clinical Oncology& Nuclear Medicine and of Obstetrics & Gynecology, Zagazig University Hospitals Conventional transvaginal ultrasonography for detection of the uterine size, endometrial thickness as basal ultrasound and followed up every six months. Diagnostic hysteroscopy and endometrial biopsy were done only for patients with abnormal uterine bleeding and asymptomatic patients with increased endometrial thickness >5mm in postmenopausal or >8mm in premenopausal cases. Results: At the end of 48-months follow-up period, the most common endometrial lesions of group I were endometrial hyperplasia in 5 patients (31.3%), endometrial polyp in 4 patients (25 %) and endometrial atrophy in two patients (12.5%). This gave impression that Endometrial polyps & endometrial hyperplasia were the most common endometrial lesion of group I. In group II the most common endometrial lesions detected by hysteroscopy and histopathological examination were endometrial atrophy in 3 patients (30%), endometrial polyp in one patient (10%) and endometrial hyperplasia in one patient (10%). Conclusions: Tamoxifen was associated with development of various endometrial changes, including endometrial hyperplasia, cystic atrophy, leiomyoma, endometrial carcinoma and other types of uterine malignancy, especially in postmenopausal patients. Even if they are asymptomatic, these patients must be evaluated carefully