ABSTRACT: Tamoxifen, a nonsteroidal antiestrogen agent, is widely used as adjunctive therapy for women with breast cancer, and it has been approved by the U.S. Food and Drug Administration for adjuvant treatment of breast cancer, treatment of metastatic breast cancer, and reduction in breast cancer incidence in high-risk women. Tamoxifen use may be extended to 10 years based on new data demonstrating additional benefit. Women taking tamoxifen should be informed about the risks of endometrial proliferation, endometrial hyperplasia, endometrial cancer, and uterine sarcomas, and any abnormal vaginal bleeding, bloody vaginal discharge, staining, or spotting should be investigated. Postmenopausal women taking tamoxifen should be closely monitored for symptoms of endometrial hyperplasia or cancer. Premenopausal women treated with tamoxifen have no known increased risk of uterine cancer and require no additional monitoring beyond routine gynecologic care. Unless the patient has been identified to be at high risk of endometrial cancer, routine endometrial surveillance has not proved to be effective in increasing the early detection of endometrial cancer in women using tamoxifen and is not recommended. If atypical endometrial hyperplasia develops, appropriate gynecologic management should be instituted, and the use of tamoxifen should be reassessed.