Moyamoya disease was originally defined as a characteristic syndrome of recurrent headaches, occlusion of the distal internal carotid arteries and the foggy (moyamoya) clusters of collateral vessels at the base of the brain as demonstrated by cerebral angiography. The etiology is unknown and pathobiology is poorly understood. We examined the intracranial arteries in 3 patients to demonstrate characteristic changes and to obtain a better understanding of the basis mechanisms of the disease. Controls were obtained from 3 normotensive patients who died as a result of cancer. Occluded internal carotid arteries were characterized by severe thickening of the intima with a dense luminal array of smooth muscle cells, a deeper less cellular zone, pronounced tortuosity of the internal elastica and thinning of the media. Collateral vessels were arterial in structure and were affected by similar proliferative changes in the intima, thinning of the media, and contorted internal elastica. Stainable lipids were not part of the typical components. Severe contortion of the internal elastica, medial damage and intimal proliferation may result from recurrent and sustained spasticity of the cerebral arteries. The distal lenticulostriate arteries showed severe medial damage similar to what is termed as a moth-eaten change in hypertensive patients dying of massive cerebral hemorrhage.