We report here the effect of diet and social environment on the carotid bifurcation atherosclerosis of 60 adult male cynomolgus monkeys (Macaca fascicularis) fed either an atherogenic diet (AD) or a more "prudent" diet (PD). Animals within each dietary condition lived either in unstable social groupings (which had their memberships reorganized on a regular basis) or in stable groupings. The experiment lasted 22 months, after which animals were necropsied and the extent of atherosclerosis was measured both at the carotid bifurcation and at other arterial sites. Carotid bifurcation atherosclerosis (CBA) was significantly more extensive and severe among animals fed the AD diet. Among these animals, all of which were normotensive, lesion extent was positively associated with both plasma cholesterol concentrations and heart rate response to behavioral stress, and negatively associated with the ability to clear glucose. No experimental variables correlated with the CBA of the PD animals; these lesions, some of which were extensive, were probably pre-existing. The social conditions and individual behavior patterns of the animals had no effect on CBA in either group, despite a pronounced effect on the coronary arteries (CA) of the same animals. While the overall correlation between CBA and CA atherosclerosis was not high, animals with extensive CBA almost always had extensive CA atherosclerosis. The factors influencing CBA in the cynomolgus monkey may be similar to those affecting humans.