Experimental studies and epidemiologic data both indicate that high-fat diets increase the risk of cancer at sites such as breast, colon, and pancreas. Dietary polyunsaturated vegetable oils promote tumorigenesis in animals whereas saturated fats and polyunsaturated fish oils either have little effect or are inhibitory. A blend of fats typical of the American diet enhanced mammary tumorigenesis effectively when fed as 40% of calories. After 9 to 10 weeks on this diet, reducing the fat to 10% of calories inhibited subsequent tumor development. Although dietary fat appears to act as a promoter, the exact mechanisms are still unknown. Cancer mortality in humans correlates better with total dietary fat than with degree of unsaturation, perhaps because most diets contain more than the amount of polyunsaturated fatty acids required for maximum effect on tumorigenesis in animals. A reduction in total dietary fat is recommended to reduce the current high cancer mortality.