Nineteen fatal cases of occlusion of the circle of Willis, so-called Moyamoya disease, were examined clinicopathologically. Fresh and massive cerebral haemorrhage was confirmed in 14 and cerebral infarcts of 4 of 19 patients. Among these 14 patients, massive haemorrhage was found in the basal ganglia, thalamus and hypothalamus of 9, and in the thalamus, cerebral peduncle and midbrain of 5. Pathologically, fibrosing stenoses or occlusions involved the circle of Willis and its major branches in all cases. In 13 of 17 patients numerous collateral channels, muscular in type, paralleled the circle, bypassing the occluded natural passages, Rupture of dilated small muscular collateral arteries was demonstrated in fresh and old haemorrhagic lesions in 3 of the 14 patients. Saccular aneurysm of cerebral arteries in the subarachnoid spaces was present in two of the 19. No rupture involved the perforating arteries in the subarachnoid space. These findings strongly suggest that in patients with Moyamoya disease rupture of overgrown perforating arteries as collaterals in brain may be main cause of single or repeated cerebral haemorrhage. Stenoses or occlusions of these perforators are presumably an important factor in the occurrence of cerebral infarcts.