Changes in arterial shear stress induce functional and structural vasculature adaptations. Recent studies indicate that substantial retrograde flow and shear can occur through human conduit arteries. In animals, retrograde shear is associated with atherogenic effects. The aim of this study was to examine the impact of incremental levels of retrograde shear on endothelial function in vivo. On 3 separate days, we examined bilateral brachial artery flow-mediated dilation, an index of NO-mediated endothelial function, in healthy men (24+/-3 years) before and after a 30-minute intervention consisting of cuff inflation to 25, 50, or 75 mm Hg. Cuff inflations resulted in "dose"-dependent increases in retrograde shear rate, compared with the noncuffed arm, within subjects (P<0.001). Flow-mediated dilation in the cuffed arm did not change in response to the 25-mm Hg stimulus but decreased significantly after both the 50- and 75-mm Hg interventions (P<0.05). The decrease in flow-mediated dilation after the 75-mm Hg intervention was significantly larger than that observed after a 50-mm Hg intervention (P=0.03). In the noncuffed arm, no changes in shear rate or flow-mediated dilation were observed. These results demonstrate that an increase in retrograde shear rate induces a dose-dependent attenuation of endothelial function in humans. This finding contributes to our understanding regarding the possible detrimental effects of retrograde shear rate in vivo.