In 2010, the American College of Cardiology Foundation and American Heart Association could not recommend brachial artery percentage flow-mediated dilation (FMD%) for risk assessment of coronary artery disease (CAD) in asymptomatic adults. We aimed to scrutinise past and recently published findings regarding FMD% in this same context of clinical utility and conclude that (1) the question of whether brachial FMD% is a suitable substitute for coronary vasodilation is addressed by method agreement statistics rather than the correlation coefficients that have been reported in past studies. Also, the much-repeated view that brachial FMD% and coronary vasodilation are "closely related" is not entirely justified, even before the influence of baseline lumen diameters on this relationship is accounted for; (2) along with the specialist training and the considerable time (≥1 h) that is required for the FMD% protocol, the error in individual measurements and population reference ranges is too large for clinical decisions to be robust on individual patients; (3) many interventions that are proposed to change FMD% also change baseline artery diameter, which can bias estimates of any intervention effects on the flow-mediated response per se, and (4) the FMD% index generates spurious correlations between shear rate, artery diameter and endothelial function, which may help to explain the apparent paradoxes of FMD% being higher in obese people and lower in athletes. In conclusion, the clinical relevance of brachial artery flow-mediated dilation is unclear at present. The dependence of the chosen index, FMD%, on initial artery size has contributed to this lack of clarity.