To study the effects of dietary and social manipulations on lesion progression in male monkeys with established atherosclerosis, 83 animals fed a diet containing 1 mg cholesterol per kcal for 14 months were either necropsied (baseline group, n=21) or assigned to one of three experimental conditions: 1) a diet containing a high amount of fat and cholesterol and a stressful social situation (HiFC-stress, n=18); 2) a diet lower in fat and cholesterol and a stressful social situation (LoFC-stress, n=21); or 3) the low-fat, low-cholesterol diet and a nonstressful social situation (LoFC-no stress,n=23).After 28 months, all animals were necropsied. Coronary atherogenesis was arrested among monkeys in the LoFC-stress and LoFC-no stress conditions compared with that of animals in the baseline condition (plaque areas of 0.35 mm2, 0.30 mm1, and 0.38 mm1, respectively). Lesions in animals fed the LoFC diet (both stress and no-stress groups) were significantly smaller than those in monkeys in the HiFC-stress condition (0.96 mm1). Furthermore, aortic cholesterol content was significantly decreased and luminal areas were relatively larger among monkeys in both LoFC conditions compared with animals in the baseline and HiFC-stress conditions (p<0.05 for all). The results demonstrate that a low-fat, low-cholesterol diet can halt plaque development, reduce arterial cholesterol content, and permit compensatory arterial enlargement, processes that were unaffected by social stress in this investigation.