There is an unusual vascular network at the base of the brain in patients with moyamoya disease. We detected various histologic lesions in the perforating arteries of 22 patients. Vessels showing rupture ranged from 50 to 530 microns in diameter; they were dilated, some had fibrin deposits in the wall, fragmented elastic laminae and attenuated media. Non-ruptured perforating arteries (diameter 200 to 550 microns) revealed microaneurysm formation, focal fibrin deposits and marked attenuation of the wall thickness with diminution of the elastic lamina. These changes seem to predispose to rupture of perforating arteries. Stenotic changes such as fibrous intimal thickening, collapse of the lumen and thrombosis were detected in 14 out of 22 cases. Morphometric analysis of perforating arteries indicated that arteries showing extreme degrees of stenosis or dilatation were more frequent in the patients with moyamoya disease than in the control cases. Dilative arteries were more frequent in the young patients and stenotic vessels were, in contrast, less frequent in the young patients.