A number of risk factors for breast cancer are considered jointly in one pathogenetic framework which relates to the formation of pre-cancerous lesions. Energy-rich diet during puberty and adolescence enhances the occurrence of pre-cancerous lesions in the breast. This process is counteracted by full-term pregnancies, and the earlier they come the fewer the number of such lesions. Energy-rich diet later in life contributes to the occurrence of obesity which, after menopause, enhances the growth of existing subclinical and clinical breast cancer. In both periods of life the nutritional factor exerts its effect through endocrine mechanisms in which oestrogens play a major part.