We describe the vascular pathologic findings in a 20-year-old Japanese man with Down's syndrome (DS) who developed stroke-like symptoms and died of respiratory failure attributed to a cerebral infarction. The latter was verified at autopsy, which also revealed vascular abnormalities resembling those of moyamoya disease. There were occlusions or stenosis in the bilateral internal carotid arteries, middle cerebral arteries, left posterior communicating artery, and basilar artery due to fibrous hypertrophy of the arterial intimal walls. The tunica media was highly atrophic and the internal elastic lamina was tortuous and duplicate. However, unlike moyamoya disease, main trunks of the cerebral arteries were of normal size, and there was marked stenosis of the basilar artery. These cerebrovascular abnormalities may have been the result of an immune deficiency, thrombosis, maldevelopment of the skull and brain, or may have been linked to moyamoya disease. Further study of similar cases is necessary to clarify the association between DS and moyamoya disease, as well as the pathogenesis of that disease.