The middle aortic syndrome, with diffuse narrowing of the thoracic and abdominal aorta, was present in 10 of 18 patients with Williams' syndrome (55%). There were 3 thoracic coarctations, and 2 abdominal coarctations, with gradients greater than 20 mmHg across the zone of narrowing. Seven patients had mild renal arterial stenosis, and 6 had visceral arterial stenoses. Ten were hypertensive. Measured dimensions of the aortic lumen failed to increase with age in 3 males who had serial angiographic studies. One developed mesenteric arterial stenosis, with mild bilateral renal arterial stenoses, between the ages of 9 and 19 years. Aortic intravascular ultrasound performed in 2 patients confirmed abnormally thick vessel walls with small lumens. Diffusely narrowed and thick-walled stiff arteries, lacking elastin, are a feature of Williams' syndrome. The arteriopathy tends to progress with age, and systemic hypertension is common in teenagers and beyond. The middle aortic syndrome was present in more than half our patients, and does not necessarily reflect a bias because of cardiologic referral. Aortography with measurement of aortic diameters and delineation of the visceral branches is an important requirement for complete evaluation of patients with Williams' syndrome.