The results of three consecutive experiments are reported in which atherosclerotic lesions of adult male rhesus monkeys produced by a ration relatively rich in calories, cholesterol, and saturated fats and resembling the "average American" table-prepared fare were compared with the atherosclerotic lesions in similar monkeys fed a low-fat, low-cholesterol, and low-calorie, so-called "prudent" table-prepared ration. Each experiment lasted 2 years. The frequency and severity of gross and microscopic aortic atheromatous disease and microscopic coronary artery disease were compared. In addition, the qualitative features of the lesions, several types of analyses of blood lipids, and the reactions of aortic cells to in vivo deposited interstitial aortic deposits of lipoprotein fractions prepared from these animals are described. In general, the animals fed the average American ration had serum cholesterols that were consistently higher (383 +/- 35 mg%) than animals fed the prudent ration (199 +/- 13 mg%). The contrasts in lesion involvement varied from about 6:1 in severity for both aortic and coronary lesions to approximately 3:1 or 4:1 in frequency. The aortic gross surface area involvement at autopsy was 46% for monkeys fed the average American ration compared to 7% for the prudent diet group. In lesions of animals fed the average American ration much of the lipid was extracellular and both cell proliferation and fiber protein deposition were prominent. The small lesions in the animals fed the prudent ration were much more likely to be composed mostly of "foam cell" intimal thickenings. The lesions seen in the animals fed the average American ration resemble those seen in autopsies of many young Americans whose lesions have been studied recently and who demonstrate raised lesions in their coronary arteries and aortas in the third decade.