Veins and arteries have active endothelium, producing vasoactive substances like nitric oxide. The aim of this study was to evaluate whether hypertensive patients exhibit venous endothelial dysfunction and to determine the relationship between endothelial-dependent and endothelial-independent vasodilation responses in venous and arterial systems. Sixteen unmedicated patients with stage I and II hypertension and without other risk factors and 15 matched normotensive volunteers had venous and arterial endothelial function evaluated with the dorsal hand vein technique and brachial artery ultrasonography. Hypertensive patients had a marked reduction of maximum dilation to acetylcholine (54.9% +/- 21.6%) compared with normotensive controls (85.2% +/- 27.0%). The flow-mediated dilation responses were reduced in hypertensive patients compared with controls (6.6% +/- 3.3%vs 12.4% +/- 2.6%, respectively). The responses to nitric oxide were similar in both groups, and the responses with the dorsal hand vein technique and flow-mediated dilation agreed in both groups. Hypertensive patients had an attenuated endothelial dependent response, indicating that endothelial dysfunction is also present in the venous system.