Evidence that dietary fat has an influence on carcinogenesis comes from both epidemiological data and experiments with animals. The experimental studies have indicated that dietary fat acts primarily as a promoter of carcinogenesis and that the effect depends on the type as well as the amount of fat in the diet. Vegetable oils containing polyunsaturated fatty acids of the linoleic acid family (n-6) have been shown to enhance mammary tumorigenesis, but a fish oil containing polyunsaturated fatty acids of the linolenic acid family (n-3) had an inhibitory effect at higher levels of intake. These and other findings suggest that the effect may be related to prostaglandins or other biologically active products of polyunsaturated fatty acids. Epidemiological data show a positive correlation between dietary fat and mortality from cancer at various sites, and this is supported by results of animal experiments in the case of colon cancer and pancreatic cancer as well as breast cancer. In the epidemiological data, cancer mortality shows strong positive correlations with total dietary fat and with animal fat, but not with fat derived from plants. Fats and oils used as spreads, cooking fats, and salad oils are the main source of fat in the American diet. Other major sources are meats and dairy products. Fat intake could probably be reduced substantially without serious deleterious effects, and this might help to decrease the risk of developing certain types of cancer.