Vascular endothelium function plays a pivotal role in cardiovascular pre-clinical markers. Nitric oxide secreted by the vascular endothelium has a vasodilatory effect that reduces blood pressure which, in turn, prevents increases of left ventricular mass and intima-media thickness and improves left ventricular relaxation. Modern ultrasound technique allows noninvasive assessment of endothelial function and permits identification of functional alteration prior to manifestation of anatomical changes in the cardiovascular system. However, effort is needed to improve the reliability and reproducibility of ultrasound derived measure of endothelial function. This article provides an overview of challenges of clinical determination of endothelium-dependent arterial dilatation. In addition, we highlight the necessity of targeting the pediatric population in studies aimed at preventing and treating cardiovascular disease.