Background. Endothelial dysfunction is common in end-stage renal disease and may contribute to the development of hypertension and atherosclerosis. However, it is not clear whether hemodialysis per se acutely affects endothelial function. Therefore we determined endothelium-dependent, flow-mediated dilatation of the brachial artery as marker of endothelial function immediately before and after hemodialysis(HD). Methods. Twenty-one chronic HD patients(12 men, 9 women; age 59짹9 years; time on dialysis 33짹4 months; blood pressure 145짹2/84짹1 mmHg) were included. Endothelial function was assessed before and after a single dialysis treatment. To estimate the endothelial function, Flow-mediated dilatation(FMD) and nitroglycerin-induced, endothelium-independent vasodilatation(NMD) were measured in brachial artery by using vascular ultrasound. Pre- and post-dialysis clinical parameters as body weight, blood pressure and heart rate were evaluated. Plasma concentrations of BNP were measured at pre-and post-dialysis. Results. FMD, reflecting the endothelial function and responsiveness to blood flow, enhanced after HD(from 4.5짹2.0% to 9.0짹4.0%: P< 0.001), where as NMD, reflecting the vasodilatation by smooth muscle, was not changed significantly(from 1.1짹5.8% to 1.4짹4.9%: P=NS). Clinical parameters as systolic BP(from 139.1짹32.3mmHg to 145.9짹39.3mmHg: P=NS), diastolic BP(from 81.4짹17.9mmHg to 88.7짹31.3mmHg: P=NS) and HR(from 68.9짹13.4/min to 70.9짹13.7/min: P=NS) were not changed, but body weight(from 57.0짹12.0kg to 55.2짹11.2kg: P<0.05) decreased. Plasma level of BNP level(from 1028짹816pg/ml to 884짹725pg/ml: P<0.05) was decreased significantly after HD. Conclusions. Endothelial-dependent vasodilatation, estimated by brachial artery FMD, was improved after HD in patients on maintenance dialysis therapy. This effect of HD may be related to improved hemodynamic loading by decreased intravascular volume and enhanced cardiac performance reflected by decrement of BNP level. These implies the need of early hemodialysis to prevent the progress of endothelial dysfunction and atherosclerosis Keywords: endothelial dysfunction; flow-mediated vasodilation; hemodialysis; BNP