AIMS: To determine whether impaired brachial endothelial (flow-mediated dilation, FMD) and smooth muscle function (nitroglycerin-mediated dilation, NTGMD), and remodelling of the common carotid artery (CCA) develop before puberty in obese children. METHODS AND RESULTS: Arterial intima-media thickness (IMT), FMD and NTGMD were measured by high-resolution ultrasound in 48 obese and 23 lean pre-pubertal children (8.8 +/- 1.5 years old). We assessed central pulse pressure, incremental elastic modulus (Einc), casual and ambulatory systolic (SBP) and diastolic blood pressure (DBP), and body fatness by DXA. Obese children had significantly lower FMD (4.5 +/- 4.0 vs. 8.3 +/- 1.7%), NTGMD (19.0 +/- 9.0 vs. 25.8 +/- 6.1%), and increased Einc (13.9 +/- 5.2 vs. 10.4 +/- 5.2 mmHg/10(2)), ambulatory SBP (121.3 +/- 12.6 vs. 106.6 +/- 7.1, mmHg), and DBP (69.1 +/- 5.7 vs. 63.7 +/- 4.5) than lean subjects, whereas IMT was not augmented. Ambulatory systolic hypertension was present in 47% of obese subjects. FMD, NTGMD, and Einc were correlated with body fatness, body mass index, and blood pressure (BP). CONCLUSION: Impaired endothelial and smooth muscle functions and altered wall material develop before puberty in obese children, however remodelling of the CCA is not yet present. Arterial dysfunction may be considered as the first marker of atherosclerosis and is associated with elevated BP. Ambulatory blood pressure monitoring may be a potential tool to improve risk stratification in obese children.