Good reasons exist to reduce intake of animal and partially hydrogenated fat. Existing data, however, provide little support for the hypothesis that reduction in dietary fat composition, even to 20% of energy during adulthood, will lead to a substantial reduction in breast cancer in Western cultures. Some evidence suggests that substituting monounsaturated fat for other sources of energy may even reduce risk of breast cancer; this deserves further examination. Also, the avoidance of weight gain during adulthood has many benefits, which are likely to include an important reduction in risk of postmenopausal breast cancer.