Objective We assessed the effect of type 2 diabetes mellitus and obesity on flow-mediated dilation (FMD) and endothelial-dependent vasodilation and carotid intima-medial thickness (cIMT) in young people.Study design Adolescents were recruited in 3 groups: subjects with type 2 diabetes mellitus (n = 15), subjects who were obese and non-insulin resistant (n = 13), and lean control subjects (n = 13). Body mass index was similar in subjects with obesity and subjects with type 2 diabetes mellitus, but higher compared with that of lean control subjects (both P <.001). Brachial artery FMD and cIMT were assessed by using Duplex ultrasound scanning imaging.Results There were no significant differences in brachial or common carotid arterial diameters in the groups. cIMT was significantly greater in the group with type 2 diabetes mellitus (0.54 +/- 0.01mm) compared with both the lean control (0.46 +/- 0.02 mm, P <.001) and obese control (0.46 +/- 0.02 mm, P <.01) groups. FMD was significantly decreased in the group with type 2 diabetes mellitus (7.98% +/- 0.54%) compared with the lean group (10.40% +/- 1.00%, P <.05).Conclusions Measures of vascular health were impaired in adolescents with type 2 diabetes mellitus compared with lean and obese adolescents who were not insulin resistant. Measures of arterial function and structure may provide pre-clinical measures of cardiovascular disease in young people at elevated risk. (J Pediatr 2011;159:971-4).