Objectives. We used intraluminal ultrasound imaging to pro vide additional information about the vascular pathology in Williams syndrome. Background. The cardiovascular pathology of Williams syndrome consists of medial hypertrophy in both systemic and pulmonary arteries, which results in lumen narrowing. Methods. Systemic and pulmonary arteries were examined in vivo using intravascular ultrasound imaging (5F, 30-MHz catheter) in three children with Williams syndrome. Results. All arteries exhibited severe wall thickening with secondary lumen narrowing. Balloon dilation of a branch pulmonary artery in two children resulted in a significant localized increase in lumen caliber associated with a tear in the vessel wall. Conclusions. Intravascular ultrasound imaging in patients with Williams syndrome may permit better understanding of the pathophysiology of the syndrome and a more rational approach to therapeutic interventions.