The detailed localization of early atherosclerotic lesions in arterial bifurcations was studied in 33 brachiocephalic arteries obtained from autopsies of young persons who had suffered a violent and sudden death. Drawings of the sudanophilic lesions in the vessel wall were fed to a computer, scaled to a standardized size and shape, and added together. The results are presented as contour lines connecting points with equal frequency of early lesions. The distribution of the early lesions reveals a distinct pattern: The early lesions start to develop on the outer walls of the bifurcation, while the inner walls downstream from the flow divider are kept free as are also the lateral walls. These results are clearly in conflict withFry'sshear stress hypothesis, while they seem to supportCaro'shypothesis of inhibition of local diffusional transport. The results are also at variance with results obtained in most animal models, suggesting that the process which takes place in the vessel wall in these animals may be different from human atherosclerosis.