That dietary fat can significantly affect mammary tumorigenesis in mice and rats has been clearly established. The purpose of this communication is to review and critique this interesting and potentially important relationship. This review focuses on the relationship between the amount and type of dietary fat and the role of calories in rodent mammary tumor development and metastasis. Additionally, the influence of dietary fat on development of human breast carcinoma transplants in immunodeficient mice is examined. The numerous studies cited in this review provide a compelling biological foundation for a potentially important relationship between dietary fat and/or calorie consumption and breast carcinoma development in human populations.