The metabolic syndrome (MetS) is characterized by abdominal obesity, elevated blood pressure, hypertriglycemia, low highdensity lipoprotein (HDL) cholesterolemia, and hyperglyceridmia (1). Vascular dysfunction caused by endothelial dysfunction is an early abnormality in the MetS that may contribute to premature atherosclerosis (2). Each of the components of the MetS has been independently associated with vascular dysfunction (3-5). Because there was the 2.6-fold increased risk of coronary death among those with the MetS (6), other mechanisms may contribute to the potential adverse effect of features of the MetS on vascular function and, in particular, endothelial function, thereby increasing the potential for atherothrombotic complications (7). Several studies demonstrated the relationship between MS and coronary microvascular circulation. MetS patients showed impaired response of myocardial blood flow to a cold press or test in by PET analysis (8), indicating the presence of coronary microvascular endothelial dysfunction. Similarly, MetS patients showed an impaired coronary blood flow in with angiographically normal coronary arteries by Thrombolysis in Myocardial Infarction frame count method (9). Moreover, patients with MetS showed an impaired vasodilatory response to pharmacologic agents in the left ascending of coronary artery by transthoracic echocardiography (10). Possible mechanisms for impaired coronary vasodilation in patients with MetS are insulin resistance which may induce endothelial dysfunction mediated by oxidative stress (11), reduced adiponectin which have a role in the phosphorylation of eNOS (12), release of proinflammatory cytokines (e.g., interleukin-6, tumor necrosis factor-α) by visceral adipose tissue (13). These mechanisms may solely and/or multifactorially contribute to coronary endothelial dysfunction in MetS patients (14).The present study revealed that coronary flow reserve (CFR) is impaired in patients with MetS and there is an independent relationship between impaired CFR and increased aortic stiffness evaluated by echocardiography. This study demonstrates that CFR was significantly correlated with age, blood pressure, waist circumference, lipid profile, fasting glucose, high-sensitivity C-reactive protein, left ventricular (LV) mass index , and aortic stiffness. The relationship between CFR and aortic stiffness persisted after adjusting for age and each components of MetS.In this study, the evaluation of aortic stiffness was performed via relating change in diameter of aorta to distending pressure, not assessed by pulse wave velocity (PWV) (15). In previous studies, significant correlations between CFR and aortic stiffness assessed by PWV have been demonstrated in patients with hypertension and coronary artery disease. PWV is the gold standard of aortic stiffness and easily performed in clinical practice. To prove the efficacy of newly non-invasive method to detect the aortic stiffness, the description of the method is needed, but the authors did not describe the methodological re...