We reported an autopsy case of Down's syndrome with moyamoya syndrome. A 
30-year-old male with Down's syndrome suffered from a cerebral infarction and 
died of brain herniation. Cerebral angiography showed vascular abnormalities 
that were the same as moyamoya disease. Pathological findings revealed multiple 
stenosis of main trunk of the cerebral arteries. Pathologically, the stenosed 
vessels showed eccentric intimal thickness with cholesterin deposit, unlike 
moyamoya disease. There are only two previous reports of autopsied cases of 
Down's syndrome with moyamoya syndrome. We postulate that a protein encoded on 
chromosome 21 may be related to the pathogenesis of Down's syndrome with 
moyamoya syndrome.