Objective: The endothelial dysfunction is thought to be an early precursor of vascular/renal damages. The present study was conducted to examine whether endothelial dysfunction, as assessed by measurement of flow-mediated vasodilatation (FMD), is longitudinally associated with the progression of vascular/renal damage, and also to identify factors that can perpetuate endothelial dysfunction in treated hypertension. Methods and Results: A 3-year multicenter prospective observational study was conducted in 674 Japanese patients with hypertension receiving antihypertensive medication, in whom the carotid intima-media thickness/plaque (CIMT), brachial-ankle pulse wave velocity (baPWV), estimated glomerular filtration rate (eGFR), urinary albumin/creatinine excretion ratio (UACR) and FMD were measured thrice at 1.5-year intervals. A lower value of the FMD at the study baseline was associated with a higher rate of increase of the baPWV during the study period (The change of baPWV during the study period was -25 cm/sec in subjects with the highest quartile range of FMD vs. +18 cm/sec in subjects with the lowest quartile range of FMD, p<0.05). However, no association of the FMD at the study baseline was observed with any of the other markers of vascular/renal damage examined. Mixed model linear regression analysis revealed a significant inverse relationship of the FMD at each measurement point with the baPWV at each measurement point (beta estimate = -10.97, P < 0.01); furthermore, the serum level of low density lipoprotein cholesterol (LDLC) at the study baseline was also inversely associated with the longitudinal changes of the FMD values (estimate = -0.39, P < 0.01). Conclusion: Our results suggest that an “endothelial dysfunction-arterial stiffening continuum” may exist in treated hypertension, and endothelial dysfunction, as measured by FMD, may be more closely associated with the arterial stiffness than with other parameters of preclinical renal/vascular damage. Furthermore, inadequate control of LDLC may also contribute to the progression of endothelial dysfunction.