Objectives We investigated the effect of hypoxia on endothelial function in adults with congenital heart disease. Background: There is growing evidence that endothelial dysfunction, which is often defined as the decreased synthesis, release, and/or activity of endothelial-derived nitric oxide (NO), is an important factor leading to vascular damage such as atherosclerosis. There are no available data about endothelial dysfunction in adults with congenital heart disease who were exposed to hypoxia. Patients and Methods Three groups were classified according to the underlying disease and hypoxia (ASD 13(M/F=8/5), TOF 8(M/F=7/1), Eisenmenger syndrome 8(M/F=5/3)) Endothelium dependent vasodilation was assessed by measuring the change in the diameter, flow velocity, and flow volume of the brachial artery at baseline, after 60 s of reactive hyperemia, relative to baseline measurements after deflation of a forearm cuff inflated to 250 mm Hg for 5 min, and endothelium independent vasodilation was assessed after sublingual nitroglycerin. Results In all three groups, the diameter of brachial artery was increased significantly in response to nitroglycerin. However, TOF group showed poor response of vasodilation to hyperemia after temporary arterial occlusion. Although the flow velocity of brachial artery was increased significantly after reactive hyperemia in all three groups, nitroglycerin could not bring a significant increase of flow velocity compared to the baseline in all. After reactive hyperemia, flow volume increased several times as those of baseline among all groups. But, flow volume did not increased in Eisenmenger group. Comparing three groups, the change of diameter showed significant difference between ASD and Eisenmenger groups(24.42짹7.19% vs. 18.45짹6.8402 %, p<0.05) Conclusions This study suggests that the exposure to hypoxia and its duration could contribute a harmful effect on endothelial function in adult with congenital heart.