The diet-heart hypothesis, that high dietary saturated fat and cholesterol intake increase the risk and high polyunsaturated fat reduces the risk of coronary heart disease in man is supported by ecologic studies, by experiments in rodents and non-human primates, by voluminous literature relating dietary factors to serum lipids, by several secondary prevention trials, and by the Lipid Research Clinics Trial demonstrating a reduction in coronary heart disease among participants assigned to cholestyramine.