The hemodynamic concept of atherosclerosis considers the laws of fluid dynamics as the primary factor in the pathogenesis of atherosclerosis. The laws of fluid mechanics can account for the localization, progressive development, and varied pathologic appearance of atherosclerotic lesions at specific areas of predilection. These are areas characterized by curvature, bifurcation, branching, tapering, or external attachment. Such locations are subject to a relative decrease in local lateral pressure in accordance with the laws of fluid mechanics. The contributing atherogenic influence of other factors, such as sex, race, diet, nutritional status, habitus, lipid metabolism, drugs, hormones, associated diseases, hypertension and stress, requires further evaluation. It appears that these are secondary factors which may modify the primary hemodynamic mechanism. The Hemodynamic Mechanism Model hydraulic systems based upon specifications of actual autopsy specimens demonstrate that sites of relative decrease in lateral pressure are sites of predilection. The incidence and degree of atherosclerosis at