Abstract Background:This study was designed to determine the short and long term effects of grape and pomegranate juices consumption on the endothelial function in adolescents with metabolic syndrome. Methods:This randomized controlled clinical trial was conducted in 2008 among 30 adolescents, aged 12-15 years, with metabolic syndrome as defined by the International Diabetes Federation. After baseline testing, the children were randomly assigned into two groups. The children of one group were asked to drink 18 ml/kg/day of natural grape juice and those in the other group were asked to drink 240 ml/day of natural pomegranate juice for 1 month. The brachial arterial reactivity was measured by high-resolution B-mode ultrasound images. At rest basal brachial dimension (BBD), 30 and 90 seconds after cuff deflation in response to reactive hyperemia, endothelium-dependent dilation (EDD) or flow mediated dilation (FMD), again at rest, and 3-4 min after receiving 400 µg sublingual nitroglycerin (TNG) producing endothelium independent dilation (EID). The sonographic evaluation was performed before drinking juice and was repeated 4 hours and 1 month after regular drinking juice, to assess short-and long-term effects of juice on endothelial dilation, respectively. Findings:FMD 30";;;;;;;;;;;;;;, 90";;;;;;;;;;;;;; and post TNG significantly improved at 4 hours after drinking juice in both groups. FMD 30";;;;;;;;;;;;;;, 90";;;;;;;;;;;;;; and post TNG also significantly improved at 1 month of drinking in both groups, but BBD was significantly different at 1 month only in grape juice group. Conclusion:Our findings suggest that daily consumption of diets rich in antioxidants can improve endothelial function in adolescents with metabolic syndrome. Key words:Metabolic syndrome, children, vascular reactivity, antioxidants