The aortic localization of diet-induced fatty streaks in relation to focal increases in intimal permeability was evaluated in cynomoglus monkeys. Animals fed a hypercholesterolemic diet and studied at 10, 15, and 100 days had increasing intensity of Evans blue dye uptake. The overlap of fatty streaks with areas of dye intake increased as the areas of dye uptake enlarged, but all hypercholesterolemic groups showed some fatty streaks not topographically related to areas of dye uptake or flow instability. Because the upper thoracic aorta tended to show more advanced fatty streak formation dissociated from evident permeability change or hydraulic instability, it is suggested that mechanical factors associated with the geometric configuration of the descending thoracic aorta may have a significant role in the localization of some hypercholesterolemic fatty streaks.