Evidence relating dietary fat to cancer at sites such as the breast and colon is provided by experiments showing that animals fed high-fat diets develop cancer at these sites more readily than do animals fed low-fat diets and by epidemiological data from different countries showing strong positive correlations between cancer incidence and mortality, and level of dietary fat. Experiments on animals have indicated that polyunsaturated vegetable oils promote cancer more effectively than do saturated fats or polyunsaturated fish oils, whereas in the epidemiological data, total dietary fat correlates with cancer incidence and mortality at least as well as does any particular type of fat. Case-control and cohort studies have not shown strong indications of a relationship between dietary fat and cancer, perhaps because of methodological difficulties inherent in such studies. The weight of evidence continues to indicate that long-term adherence to a low-fat diet can reduce the risk of some common types of cancer.