The arterial lesions of William’s syndrome are characteristic1: supravalvar aortic stenosis, coarctation of the aorta, peripheral pulmonary artery stenosis, and stenosis of the renal and coronary arteries. The development of slowly progressive coarctation has been described in only one patient,2 ,3 in whom angiography at 3 years of age showed a diffusely hypoplastic ascending aorta but no coarctation. Seven years later, angiography showed long segment coarctation distal to the left subclavian artery. We describe a patient in whom coarctation of the aorta developed over a much shorter time period.