Atherosclerotic lesions were induced in rhesus monkeys by feeding them a high-fat, high-cholesterol diet for 2 years. Arteries were examined after autopsy of a subgroup of animals (group P) and cholesterol was removed from the diet of the remaining animals. Lesions were examined in other subgroups after 30 weeks (group R1) and after 52 weeks (group R2). A control group (group C) was fed the diet without cholesterol throughout the study. The mean total serum cholesterol concentration before, during, and after lesion induction was 151, 390, and 157 mg/dl, respectively. The mean percent of surface area with fatty streak or fibrous plaque and the free and esterified cholesterol content of the artery increased in all six arterial segments examined in group P. The means for percent of surface with fatty streak and for arterial cholesterol content or concentration (but not for extent of surface with fibrous plaque) were consistently less in groups R1 and R2 than in group P, although they remained greater in groups R1 and R2 than in group C. The mean intimal thickness for coronary arteries was 10-fold greater in group P than in group C and 60 percent less in groups R1 and R2 than in group P; there was, however, much variability among animals and these differences among groups were not statistically significant. By using several measures in several arterial systems, we have shown that there was regression of diet-induced atherosclerotic lesions in rhesus monkeys while they were fed a diet high in saturated fat but without cholesterol for 30 or 52 weeks.