Vascular endothelial injury by high shear stresses and adverse effects of low shear rates on mass transfer across the arterial wall have been suggested as factors in atherogenesis. This study describes differences in blood velocity, and therefore differences of shear rate, across the lumen of the right coronary artery (RCA) of man. Selective coronary arteriograms of 30 patients without obstructive RCA disease were reviewed. Velocity was assessed qualitatively based on the rate of clearance of contrast material. There was a rapid clearing of contrast material along the outer wall of the RCA as it curved around the border of the heart. A much slower clearing occurred along the inner wall, bordering the myocardium, which persisted 2 to 6 cardiac cycles after the outer wall had cleared. This suggests that velocity, and therefore shear rate, is much lower along the inner wall of the RCA. To determine the relation of the distribution of atherosclerotic plaques in the RCA to local blood velocity, the RCA in 17 randomly selected human subjects who died of noncardiac disease were studied histologically. There was an uneven distribution of atherosclerotic plaques in the RCA with greater involvement of the inner wall. These observations demonstrate an association between the lower shear rate along the inner wall of the RCA and the site of higher concentration of atherosclerosis.