Supravalvular aortic stenosis (SVAS) is an uncommon but well characterized congenital narrowing of the ascending aorta above the level of the coronary arteries. It can be a familial disorder, can occur sporadically, or associated with Williams syndrome (WS) which is a neurodevelopmental disorder affecting connective tissue and the central nervous system. Sudden death is a well-known complication of non-syndromic SVAS but few cases have been reported associated with WS. We present a case of sudden death in a woman with the diagnosis of SVAS and WS since the age of 3 years who refused surgical correction and died at the age of 27 years. At autopsy, the aorta and pulmonary trunk were narrowed and the walls showed peculiar microscopical characteristics. In the cardiac conduction system the His bundle was small and intramyocardial. The incidence, pathology, pathogenesis and prognosis of both conditions (SVAS and WS) are reviewed.