Flow-mediated dilation (FMD) assesses the health of the vascular endothelium. Despite widespread adoption of scaling practices in cardiac research, scaling for body size or composition has not been used for FMD. The present study investigated the relationships between brachial FMD and body composition in 129 children aged 9-10 (75 female symbol, 54 male symbol), and 50 men aged 16-49. Body composition variables (total, lean, fat mass in the whole body, arm, forearm) were assessed by dual-energy X-ray absorptiometry, FMD was measured in the brachial artery using high-resolution ultrasound. FMD was scaled using simple ratios (y/x) and allometric approaches (y/x ( b )) after log-log least squares linear regression produced allometric exponents (b). Size independence was confirmed via bivariate correlations (x:y/x; x:y/x ( b )). No relationships were evident between FMD and body composition variables in adults. Small correlations existed between FMD and measures of segmental fat mass in children (r = -0.18 to -0.19, p < 0.05), there were no significant relationships between FMD and measures of lean or total mass in children. For all significant relationships, b-exponents were different from 1 (CIs -0.36 to 0.07), suggesting ratio scaling approaches were flawed. This was confirmed when ratio scaling produced negative residual size correlations, whereas allometric scaling produced size-independent indices. Correlations between FMD and body composition were weak in children and insignificant in adults. As the results of this study are limited to the populations examined, our findings do not support the adoption of scaling procedures to correct FMD.