Tamoxifen is widely used in the treatment of breast cancer and has been proposed as a prophylactic agent in this disease. Tamoxifen is an effective antioxidant and protects membranes and low-density lipoprotein (LDL) particles against oxidative damage. This antioxidant action is shared by endogenous and synthetic oestrogens. The ability of tamoxifen to protect LDL particles against the oxidative damage implicated in atherosclerosis may be an important factor in the reported cardioprotective action of tamoxifen in women being treated for breast cancer. In addition, tamoxifen has been found to act in a similar manner to oestrogens to lower plasma cholesterol levels. The cardioprotective action of tamoxifen may be a key factor in predicting the likely risk/benefit ratio for prophylactic tamoxifen treatment in otherwise healthy women, who have been calculated to be genetically predisposed to developing breast cancer. In the future, predisposition to breast cancer may be determined by genetic screening.