Evidence is accumulating that diets rich in monounsaturated fatty acids might be of value in the treatment of hypercholesterolemia and prevention of coronary heart disease. This communication shows that such diets cause increased concentrations of liver cholesterol in rats and rabbits. It is not known whether this effect is harmful. Caution is warranted in extrapolating the animal data to man, but the possibility that monounsaturated fatty acids increase liver cholesterol in man should be considered.