Tamoxifen is the most often prescribed non steroidal antioestrogenic agent in the world for breast cancer. Worldwide collaboration. has centralized the results, of different trials throughout the world on oral adjuvant therapy in the early stages of breast cancer. A significative regression of the tumour was observed in most cases. Moreover, recent epidemiological studies suggest that tamoxifen could prevent new contralateral primary tumours. The risk of the disease should thus be reduced by the prophylactic use of antioestrogens such as tamoxifen. Investigations using a variety of models have evaluated the effect of tamoxifen on tumour promotion and cell growth. Tamoxifen-induced growth inhibition is associated with major changes in biochemical events in cultured human breast cancer cells including cell proliferation or growth factor production. Growth inhibition of oestrogen-responsive human breast cancer cells is associated with an induced secretion of autoinhibitory polypeptides (TGF beta) and an antagonistic effect on the synthesis of proliferative proteins (TGF alpha,...). The first step in the mechanism of action of the drug is binding of tamoxifen to the oestrogen receptors. Development of resistance to tamoxifen treatment is a great problem in treatment of breast cancer patients and the mechanism of resistance will require further study: under the influence of the drug, tumours could become remodelled as selected subpopulations emerge resistant-tamoxifen. The fact that some breast cancers which are oestrogen receptor-negative respond to antioestrogen suggests that parallel but separate pathways for oestrogen and antioestrogen action may exist. This paper summarizes the results of the most recent studies concerning this promising drug.