['Recent data demonstrated that serum phosphorus, within the normal range, is an independent predictor of atherosclerotic cardiovascular disease, independently of renal function. Traditional cardiovascular risk factors are important mediators of endothelial dysfunction, the early step of atherosclerosis. We designed this study to evaluate a possible correlation between serum phosphorus and endothelium-dependent vasodilation, evaluated by the strain-gauge plethysmography, in naïve hypertensives.', 'We investigated by strain-gauge plethysmography, the relationship between forearm blood flow (FBF) response to acetylcholine (ACh) and serum phosphorus in 500 patients with uncomplicated, never-treated, essential hypertension, divided by phosphorus tertiles. There were no significant differences among tertiles with the exclusion of forearm blood flow (FBF). Phosphorus (β\xa0=\xa0-0.454; P\xa0=\xa00.0001), estimated-glomerular filtration rate (e-GFR, by CKD-EPI formula) (β\xa0=\xa00.261; P\xa0=\xa00.0001), gender (β\xa0=\xa00.215; P\xa0=\xa00.0001), BMI (β\xa0=\xa0-0.086; P\xa0=\xa00.018), HDL-cholesterol (β\xa0=\xa00.077; P\xa0=\xa00.036) were significantly related to endothelium-dependent vasodilation. In an additional analysis including serum high sensitivity C-reactive protein (hs-CRP) (measured in 400 patients) in the same model, the link between serum phosphorus and ACh-stimulated FBF did not change (β\xa0=\xa0-0.422; P\xa0=\xa00.0001). Clinically relevant, 0.1\xa0mg of phosphorus increase is associated with a reduction of 22% of ACh-stimulated FBF. On multiple logistic regression analysis, the risk of endothelial dysfunction was about twice higher in patients in the second (OR\xa0=\xa01.754, 95% CI\xa0=\xa01.055-2.915; P\xa0=\xa00.030) and three-fold higher in the third tertile (OR\xa0=\xa02.939, 95% CI\xa0=\xa01.598-5.408; P\xa0=\xa00.0001) in comparison with those in the first tertile of phosphorus.'] Are serum phosphorus levels associated with endothelial dysfunction in hypertensive patients?