Introduction: Endothelial function at high altitude has been measured only in populations that are genetically adapted to chronic hypoxia. The objective of this study was to evaluate endothelial dysfunction (ED) in a nongenetically adapted high-altitude population of the Andes mountains, in Huancayo, Peru (3,250 meters above sea level). Methods: Participants included 61 patients: 28 cases and 33 controls. The cases were subjects with hypertension, diabetes mellitus, obesity, or a history of stroke or coronary artery disease. Flow-mediated vasodilation (FMD) of the brachial artery was measured in the supine position, at noon, after 5 minutes of resting. The brachial artery was identified above the elbow. Its basal diameter was measured during diastole, and FMD was tested after 5 minutes of forearm ischemia. Intima-media complex in the right carotid artery was also determined. An increase in the artery’s baseline diameter <10% indicated a positive test. Endothelium-independent vasodilation was evaluated with sublingual nitrate administration. The intima-media complex in the right carotid artery was also measured. Results: 100% of diabetics had ED; ED was also found in 68.8% of obese individuals, 55% of hypertensive patients, and 46.5% of controls. Age, height, body mass index, and waist diameter were higher in the cases as compared with the controls. A total of 57.9% (n=11) of the cases and 45.2% (n=19) of the controls presented ED. Patients without ED had a mean increase in brachial artery diameter of 23.16%, while in those with ED it was only 3.84%. Individuals with diabetes or hypertension had a greater thickness of the carotid artery intima media layer (1.092 versus 0.664 cm) (p=0.037). A positive test for ED was associated with a greater basal diameter of the brachial artery (4.66±0.62 versus 4.23±0.59 cm) (p=0.02). A total of 7 patients presented paradoxical response, developing posthyperemia vasoconstriction. Discussion: The proportion of ED was high among controls and among patients with risk factors. Controls showed better FMD profiles than subjects studied in Tibet and the Himalayas.