Postmastectomy radiotherapy is associated with a lower locoregional recurrence rate and improved disease-free and overall survival when combined with chemotherapy in premenopausal high-risk breast-cancer patients. However, whether the same benefits apply also in postmenopausal women treated with adjuvant tamoxifen for similar high-risk cancer is unclear. In a randomised trial among postmenopausal women who had undergone mastectomy, we compared adjuvant tamoxifen alone with tamoxifen plus postoperative radiotherapy.