Purpose of Investigation: To evaluate the effects of tamoxifen on the endometrium of breast cancer patients by hysteroscopy and endometrial sampling. Methods: Thirty-seven breast cancer patients using tamoxifen underwent hysteroscopy because of postmenopausal endometrial thickening or abnormal uterine bleeding. Hysteroscopic findings were compared with histopathology and ultrasonographic measurement of the endometrium. Results: Nineteen women showed endometrial abnormalities (51%) out of 37 patients. Negative and positive predictive values for hysteroscopy in detecting endometrial abnormalities were 100% and 94%, respectively. Conclusion: Endometrial surveillance is an important part of gynecological follow-up in breast cancer patients using tamoxifen and the liberal use of hysteroscopy aids in the diagnosis of abnormal endometrium.