In this study cockerels were fed varying diets supplemented with lard, vegetable oil, and cholesterol. The degree of atherosclerosis was compared in serial autopsies and only the cholesterol-fed group showed a significant difference. During a period of two years there was no apparent change in the degree of atheroma when the diet was supplemented with cottonseed oil or lard. Reversibility of the cholesterol-induced atheroma was also studied in a few chickens, and it appears that the early lesions may undergo considerable regression after cholesterol feeding is stopped. Older lesions, on the other hand, appear to be quite fixed.