Moyamoya phenomenon occurs in response to an occlusive vasculopathy affecting the distal internal carotid artery and its proximal branches. The nature of the occlusive vasculopathy is unknown in most patients. We present a patient in whom 3T magnetic resonance imaging was used to examine the arterial wall at the site of occlusion in a patient with unilateral moyamoya phenomenon. Signal characteristics were consistent with atherosclerotic disease. 3T magnetic resonance imaging may be useful for distinguishing the underlying etiology of moyamoya phenomenon in some patients.