Tamoxifen has been found to be a safe and effective treatment for all stages of breast cancer. Long term tamoxifen therapy is associated with some rare, but potentially serious, side effects so patients should be carefully monitored. However, long term tamoxifen therapy is also associated with a number of physiological benefits over and above its tumouristatic action. These benefits include a decrease in the development of contralateral breast cancer, the maintenance of bone density in postmenopausal women and a decrease in cardiovascular disease. The successful application of tamoxifen to treat breast cancer has increased enthusiasm to test its worth to prevent breast cancer. Although there are individual requests by patients for tamoxifen to prevent breast cancer, individual treatment is inappropriate. Tamoxifen can only be adequately evaluated as a preventive in randomized, double-blind clinical trials. These trials are in place and physicians should encourage women to participate and establish a new therapeutic option as rapidly as possible.