During the spring of 1993, UKCCCR prevention trial group proposed to the french medical community to participate into the international breast cancer prevention trial by tamoxifen set up in Great Britain. in January 1994 this proposal was eventually judged inacceptable in France because of lack of reflexion on primary cancer prevention medico-legal risks, tamoxifen use risks and the eligibility criterias. A randomized trial could be designed and carried out in France to evaluate the place of tamoxifen in breast cancer prevention in the following settings : post menopausal patient with atypical epithelial hyperplasia or lobular in situ carcinoma.