A case-control study has been conducted in four areas in Canada in which 400 cases of breast cancer matched by age and marital status with neighborhood controls were administered a medical and dietary history questionnaire, a 24-hour recall for dietary information and a four-day diet record. The Study has produced evidence of an association between an increased intake of nutrients, especially total fat, in both pre-menopausal and post-menopausal women with breast cancer. Reasons why a weak association might have been anticipated are discussed, and it is concluded that in reality the association is stronger. Furthermore, its consistency with other evidence, both experimental and international, suggests that it is causal.