Regression of atherosclerosis has been demonstrated in several species of animals, including rabbits, chickens, rats, dogs, pigeons, pigs, and nonhuman primates. Regression has been associated with withdrawal of cholesterol from the diet or with the ingestion of cholestyramine, alfalfa meal, or alfalfa saponins in monkeys fed high-cholesterol diets. Although regression of atherosclerosis has been documented by sequential contrast arteriography in humans, the correspondence of regression in animals and in humans needs to be established.