Objectives: Identify the prevalence of endothelial dysfunction in hypertensive patients (pts) with different degrees of severity and target organ damage (TOD), and compare the effects of angiotensin-receptor blocker (losartan) and beta-blocker (atenolol) on vascular endothelial function (EnF) in those pts.Background: High BP and angiotensin II are associated with impaired EnF. Little is known about the effects of antihypertensive therapy on EnF and the relationship between endothelial dysfunction and TOD.Methods: Brachial ultrasound was used to measure endothelium-dependent flow-mediated dilatation (FMD) and response to endothelium-independent nitroglycerin (NG) in 96 hypertensive male pts (SBP: 140-190, DBP: 90-115 mmHg). Pts have no other cardiovascular risk factors and their mean age was 49 (±5.6) years (SD). Pts were randomized to Atenolol (A) 50-100 mg/d (48 pts) and Losartan (L) 50-100 mg/d (48 pts). Thiazide diuretics were added when needed to achieve BP control. Studies of EnF and TOD (echocardiography and carotid duplex ultrasonography) were repeated after 3 and 6 months of therapy.Results: There was an inverse correlation (r=-0.41, p<0.0001) between BP and FMD before start of drugs. Both drugs produced improvement in FMD but changes with L were more significant (p<0.0001). There was no relation between extent of reduction in BP and improvement in FMD. Both antihypertensive drugs reduced left ventricular hypertrophy to a similar extent however significant reduction in carotid intima-media thickness (IMT) was noticed only with L.Conclusion: There was a relationship betweeb hypertension and endothelial dysfunction. Both losartan and atenolol improved FMD but changes with losartan were more significant. These changes did not correlate with their hypotensive action. Losartan had a favorable effect on reduction of carotid intima-media thickness and produced a comparable effect on reduction of left ventricular hypertrophy.