Fifty monkeys of the species Erythrocebus patas were fed a control monkey chow, a semi-synthetic diet containing 25% lard, or a semisynthetic diet containing 25% lard and 0.5% cholesterol for 2 years. The patas monkeys had naturally occurring atherosclerosis that was greatly accelerated by feeding a diet containing cholesterol. The atherosclerosis involved the aorta, predominantly the abdominal portion, the coronary arteries, and various peripheral vessels. Histologically, the atherosclerosis was characterized by intimal proliferative lesions associated with intra- and extracellular lipid deposition. Complicated lesions that developed after 2 years on the cholesterol-containing diet were associated with lipid crystals, necrosis, mineralization, and encroachment upon the media. Adventitial reactions characterized by increased vascularity and the presence of inflammatory cells were seen. All of these observations have been described as components of the human atherosclerotic disease process. The similarity of the patas monkey atherosclerosis to human atherosclerosis, the relatively large size and easy handling of the animals, and the fact that previous studies have shown the lipoproteins of both control and cholesterol-fed monkeys to resemble human lipoproteins all contribute to making the patas monkey a useful model for the study of experimental atherosclerosis.