Measurement of endothelial function is useful in atherosclerotic clinical research. Although optimal methodology for evaluating endothelial function has yet to be determined, it is one of the most useful methods of using ultrasonography to evaluate flow-mediated dilatation (FMD). FMD increases with increase in concentration of nitric oxide (NO), which is induced by shear stress. Impaired FMD reflects loss of NO, which correlates with early onset of atherosclerosis. FMD measurement requires a laboratory equipped with a high-resolution ultrasound linear array probe (7-15MHz) and an arm rest with a stereotactic clamp. Recently developed echo-tracking systems are preferable because they more easily visualize the vessel without producing compression and mechanical distortion. Ideally, patients should be fasting and measurements should be taken in the morning. The cuff should be placed on a radial artery; degree of cuff inflation is standardized to 50 mmHg above the patient′s systolic pressure; duration of cuff inflation should be 5 minutes; brachial artery FMD should be calculated; and nitroglycerin-mediated dilatation should be also evaluated. FMD measurements can be used to detect early coronary artery disease, cerebral artery disease, or peripheral artery disease in the clinical setting. Further, some reports indicate that FMD can be also used to predict the prognosis of cardiovascular events. In summary, FMD measurement using ultrasonographic technology provides a simple and useful way to evaluate endothelial function, which is correlated with early atherosclerosis.