Flow mediated dilation (FMD) is a noninvasive technique to assess endothelial function in humans. In most studies FMD is reported as the percent change in brachial artery diameter at 60 seconds after cuff deflation relative to baseline (%FMD at 60 seconds). Recently the availability of automated computer-based edge detection allows measurement of peak dilation during the entire time course of brachial dilation post-hyperemia. Considering that changes in arterial diameter in response to shear stress is a continuous process and that peak dilation may differ in pathological states and occur at any arbitrary time point, we examined the time course of brachial artery FMD after a 5-min period of ischemia in 38 obese African American women (42±9.3 years old, BMI 39±6, % body fat 48.2±4.3, SBP 131±13, DBP 83±8 mm Hg, HR 77±12 bpm). Results were analyzed using brachial analyzer software (Medical Imaging Applications LLC). Baseline brachial artery diameter was 3.95±0.55 mm, the time to peak was 48.93 ±13.82 seconds. The percent change in FMD (%FMD) obtained at peak diameter was greater (4.7±3.19 mm) than %FMD at 60 seconds (3.23±4.33 mm); mean difference = 1.4 (95% CI, 0.54-2.4, P=0.003). A large proportion (79%) of true peak diameters fell outside the 60 seconds time frame typically used to assess FMD, Figure. Our results suggest that the peak vasodilator effect occurs before 60±5 seconds in the majority of our population. Therefore, the evaluation of FMD as a continuous measurement has greater accuracy and less variability for the evaluation of FMD compared with an arbitrary time point.