OBJECTIVE: The aim of this study was to compare and evaluate the relationship between body composition and arterial function among youth throughout a range of body mass index (BMI) percentiles. METHODS: Ultrasonographic measurements of the carotid artery and abdominal aorta were obtained from 190 youth (96 female, 12.9 ± 0.2years). Body composition was assessed by dual X-ray absorptiometry. Participants were stratified by BMI as normal weight (NW: <85th percentile; n=79), overweight/obesity (OW/OB: ≥85th to <95th percentile; n=61), and severe obesity (SO: ≥120% of the 95th percentile; n=50). Analysis of variance and linear regression evaluated group differences and the association of body composition variables and arterial function, respectively. RESULTS: Lean mass (NW = 32.9 ± 9.2kg. OW/OB=25.1 ± 8.8kg, SO = 44.7 ± 12.5kg; p<0.001, all). and fat mass (NW = 10.8 ± 10.5 kg. OW/OB=39.3 ± 10.8kg, SO = 48.7 ± 12.6kg) were all found to be significantly higher (p<0.001, all) across increasing BMI groups. Aortic and carotid intima-media thickness was significantly increased between groups (Aortic: NW = 0.59 ± 0.09mm, OW/OB=0.51 ± 0.17mm, SO = 0.43 ± 0.17mm; Carotid: NW = 0.54 ± 0.05mm, OW/OB=0.51 ± 0.07mm, SO = 0.48mm ± 0.11; p<0.0001 for all groups). Aortic cross-sectional distensibility was significantly (p=0.033) elevated between the SO (30.61 ± 1.48%) and NW (36.39 ± 1.49%) but not observed in the carotid artery. Aortic incremental elastic modulus (IEM) was significantly greater in the SO (1153.0 ± 70.8mmHg) group compared to OW/OB (960.4 ± 48.6mmHg) and NW (846.0 ± 40.9mmHg) groups (p<0.001). Carotid IEM was significantly greater (p=0.005) in SO (1191.07 ± 383.09mmHg) compared NW (998.91 ± 313.19mmHg). LM and LMI were similarly associated with increased compliance in the abdominal aorta, but not the carotid artery. FM and FMI were similarly associated with greater arterial stiffness in both arterial beds with absolute FM showing more consistent associations with decreased vascular function. CONCLUSION: Vascular structure and function vary across BMI percentiles in children and adolescents. Lean mass and fat mass relate to both arterial structure and function differently in different vascular locations, especially in the abdominal aorta. Lean mass displays a protective vascular effect in the abdominal aorta more than the carotid while fat mass displays a negative effect in both arterial beds.