The aim of this study was to identify which phase of wall shear stress (SS) augmentation during reactive hyperemia (RH) is the crucial determinant of endothelial reactivity. Fifty-three patients with untreated essential hypertension were enrolled. A 7.5-MHz linear array transducer was used to record the 2D images and Doppler flow signals of brachial artery at baseline and during RH. Wall SS and flow-mediated vasodilation were calculated sequentially. The result of this study demonstrated that augmentation of wall SS at end-diastolic phase during peak RH is the main rheologic determinant of endothelial function in patients with untreated essential hypertension.