The atherogenicity of dietary peanut oil (PO) was examined in cynomolgus monkeys fed semipurified diets for 15 months. Four groups of six monkeys were fed diets containing 0.22 mg/kcal (0.1%) cholesterol and 0%, 5%, 10%, or 20% PO. An additional group was fed 2.0% cholesterol and 20% PO to serve as a literature control. Increasing the concentration of PO in the diet was associated with significant decreases in total plasma cholesterol (p less than 0.05) and the total/HDL cholesterol ratio (p less than 0.05) and an increase in the terminal HDL cholesterol concentration (p less than 0.05). Intimal thickness and composition were determined from cross sections of the thoracic aorta and the iliac and coronary arteries. Increasing dietary PO was associated with decreases in thickness (p less than 0.005), lipid (p less than 0.001), and smooth muscle cells (p less than 0.005) in the aortic intima. Aortic intimal lipid was positively correlated with the ratio of total to HDL cholesterol (r2 = 0.78, p less than 0.05). Monkeys fed 2.0% cholesterol and 20% PO revealed extensive atherosclerosis in all three arterial sites compared with those of any other group. Whereas the dietary fat effect was most demonstrable in the aorta, dietary cholesterol had a greater effect on the iliac and coronary arteries than it did on the aorta. Under these circumstances, dietary PO was not atherogenic in cynomolgus monkeys when fed with a concentration of cholesterol equivalent to that consumed by humans.