Incidence of breast cancer is increasing. Although modifications in breast cancer therapy were achieved, the mortality rate remained unchanged. 21 years ago, tamoxifen was used for the first time in the treatment of advanced breast cancer of postmenopausal women. In the course of time, tamoxifen therapy became the endocrine treatment of choice for selected patients in all stages of breast cancer. Besides a reduction in annual relapse and an increase in disease-free survival, it could be proven that the intake of tamoxifen over long periods of time has a beneficial effect on plasma lipid metabolism as well as bone metabolism. Comprising the beneficial effects of tamoxifen, the possibility and feasibility of breast cancer prevention in a high-risk population were to be evaluated.