An 18 year old girl with typical clinical features of Williams syndrome suddenly died of intracerebral hemorrhage due to moyamoya disease. Autopsy revealed vascular abnormalities, such as supravalvular aortic stenosis (SAS) and an abnormal complicated cerebrovascular network in the cerebral arteries. The arterial wall of the SAS lesion consisted of thickened medial tissue showing elastic disorganization with prominence of the smooth muscle cells. The narrowed vessels of the circle of Willis showed intimal thickening with an extremely wavy internal elastic lamina and marked thinning of the media. To our knowledge, this is the first report of moyamoya disease associated with Williams syndrome.