International differences in breast cancer morbidity and mortality and studies on migrating populations point to the overriding importance of environmental factors in the etiology of breast cancer. Factors directly or indirectly associated with ovarian activity do not appear to explain international differences. Population correlation studies have indicated that much of the difference appears to be explicable on the basis of nutritional factors, particularly, high total fat intake. Animal experimental studies confirm the importance of high fat diet, possibly mediated through prolactin. Other indirect support for the nutritional hypothesis is supplied by changing incidence rates in Iceland, possibly correlated with changing nutritional practices and the association of breast cancer with weight and possibly also with height. The association with height (which would suggest nutritional effects mediated through childhood diet) has not been supported by a study in Canada. Direct investigation of the association between nutrition and breast cancer is difficult because of problems in dietary methodology. Nevertheless, the results of a Canadian case-control study provide support for the importance of high fat intake. Further studies of this question are undoubtedly required, however, before specific recommendations on dietary modification can be made.