BACKGROUND Breast cancer is one of the most important public health problems facing women in the United States and similar populations today. No readily modifiable risk factors for the disease have been conclusively identified, hence establishment of a positive association between fat intake and risk is of major public health importance. METHODS An evaluation of the strengths and limitations of epidemiologic studies addressing the postulated association between fat intake and breast cancer risk is presented. In light of this assessment, an evaluation of the results of ecologic and case-control studies has been conducted. RESULTS Of the three types of epidemiologic studies considered, ecologic studies provide the weakest evidence. Although case-control studies are more prone to bias than cohort studies, there is conflicting evidence as to the existence and magnitude of bias in previously conducted case-control studies. Ecologic studies and case-control studies support the existence of a positive association, particularly for saturated fat intake in postmenopausal women. However, given the methodologic limitations of these types of study and the more equivocal nature of evidence from cohort studies, it would be imprudent to interpret these associations as representing causality. CONCLUSIONS The totality of the epidemiologic evidence does not support the existence of a strong association between fat intake and breast cancer risk; however, the possibility of a weak association cannot be excluded. Even a weak association could be important in terms of public health. The most fruitful area for future research is likely to be the establishment of new cohort studies and the continued follow-up of existing cohorts, particularly if these could address diet at different times in life, for example, during adolescence.