Breast cancer, which is the most frequently diagnosed malignancy in women today, remains a major threat. As the incidence increases at 1% to 2% annually, breast cancer strikes about 182,000 US women each year and kills 46,000. Known risk factors, such as age, early menarche, late age of first pregnancy, late menopause, family history, and obesity, account for only 40% to 50% of breast cancer cases. Consequently, the etiology of 50% to 60% of cases is unknown. Perhaps no area is more controversial than the link between nutrition and cancer, in particular the association between dietary fat and the development of breast cancer. Findings from animal research and international correlation studies indicate a causal relationship between fat intake and breast cancer. A recent analysis of 12 case-control studies among postmenopausal women also showed a 50% increase in relative risk among women ingesting high intakes of saturated fat. Furthermore, analyses of postmenopausal women in Hawaii estimated that 10% to 20% of breast cancer could be prevented by notably decreasing saturated fat intake. Nevertheless, epidemiologic studies (case-control and cohort) give conflicting results regarding the causal relationship between fat intake and breast cancer. The fact that these studies have failed to consistently show a significant association between dietary fat and breast cancer risk may be attributable to the difficulty of collecting accurate dietary information and other methodologic limitations. Findings are also limited by the lack of data on the influence of a high-fat diet during childhood.(ABSTRACT TRUNCATED AT 250 WORDS)