There are substantial data on breast tumorigenesis in animals that suggest that diet may be an important factor in human breast cancer etiology. The promotional effects of dietary fat, and, in particular, unsaturated fats, on mammary tumors in rodents is well established. The geographic distribution of breast cancer in humans correlates with international differences in average fat intake. Differences in dietary habits among populations in the United States and their breast cancer risk also have been observed. In the United States, the trend has been toward increased total fat consumption and increased use of polyunsaturated fats. However, breast cancer incidence among white women in the United States has changed very little. Case-control studies of dietary intake and breast cancer risk have shown inconsistent results, and prospective studies of breast cancer mortality and serum cholesterol and serum lipids show no differences in risk between women with high levels of cholesterol and serum lipids compared with women with low levels. Laboratory studies also suggest the possibility that natural inhibitors of breast cancer may occur in the diet as well. Antioxidants, inducers of microsomal enzyme activity, and retinoids, all have been implicated in the metabolic epidemiology of breast cancer. Research results a t Roswell Park Memorial Institute have associated lower levels of intake of dietary vitamin A with a slightly elevated risk of breast cancer. To date, the epidemiologic data do not indicate with confidence that any specific dietary risk factor may be associated with breast cancer risk in the United States population. Additional epidemiologic studies on inhibition or promotion of breast cancer following the leads of previous laboratory research may clarify the nature and practical significance of the relationship between diet and breast cancer.