Evidence for an association between colorectal cancer and breast cancer is reviewed. Consistent with a dietary hypothesis to account for a relationship between the cancer sites, intestinal and breast cancers show a similar pattern in correlations of cancer mortality and incidence rates with per capita food consumption figures. The international data indicate that meat, particularly cattle meat, is the food most closely associated with both colorectal and breast cancer rates. In the one case-control study which has shown dietary factors to be related to colorectal cancer [1], the specificity of beef as the most suspect food item implied the need for studies of long-chain, saturated fatty acids. A high animal fat and protein diet could also mean elevated levels of dietary tryptophan. Studies of tryptophan metabolism are viewed as suggestive of a biochemical mechanism for the interaction of nutritional and hormonal factors. A hormonal mechanism affected by dietary fat has similarly been postulated [2] and has some experimental support [3]. Possible research in further exploration of an association between colorectal and breast cancers is suggested.