Prevention and regression of induced atherosclerosis by d-alpha-tocopherol was investigated in 24 male M. fascicularis. One group received a basal diet, while three others consumed an atherogenic diet. Two of the latter groups also received tocopherol, one at the onset of the study (prevention) and the other after atherosclerosis was established by ultrasound evaluation (regression). Atherosclerosis was monitored over a 36-month period by duplex ultrasound imaging of the common carotid arteries. At termination, 24 arterial sites were examined for histopathology. In those animals receiving an atherogenic diet, mean percent ultrasound stenosis at 36 months posttreatment was lower in the tocopherol-supplemented groups (61 and 18%) than in the unsupplemented group (87%). Plasma tocopherol concentration was negatively correlated with percent ultrasound stenosis (p less than 0.002). Percent stenosis in the regression group decreased from 33 to 8% (p less than 0.05) 8 months after tocopherol supplementation. Although not consistently significant, histopathological changes were greater in untreated compared to treated animals. These results, if confirmed, indicate that d-alpha-tocopherol may be prophylactically and therapeutically effective in atherosclerosis.