A series of photomicrographs shows that deposits of cholesterol crystals in the walls of arteries as well as in other tissues have a strong irritative effect on collagen tissue. This process is probably the very cause of the atherosclerotic plaque. Cholesterol deposits are especially abundant in hypercholesterolemia. This may be due to various factors, the most important of which is a faulty diet containing too much saturated fat. Replacement of such fat by polyunsaturated fat will lower the blood cholesterol level. From preventive point of view, it would appear advisable to change the dietary habits of the entire population in countries with a high frequency of atherosclerotic vascular diseases.