A 13-year-old boy was admitted to our hospital with altered states of consciousness coupled with a headache and nausea. Upon admission, the patient was afebrile and comatose with a decorticated posture and was subsequently intubated. All routine laboratory tests and cerebrospinal fluid analyses were normal. Brain T2-weighted MRI (figure 1A) revealed multiple hyperintense signals in the brainstem and cerebellum. A single gadolinium-enhanced lesion was observed in the left occipital lobe. These observations were indicative of acute disseminated encephalomyelitis (ADEM) and we subsequently started methylprednisolone pulse therapy. In the follow-up MRI study, the lesions were necrotic, suggesting changes after a stroke rather than ADEM. The MR angiography (figure 1B) and the conventional cerebral angiography (figure 1C, D) performed on days 25 and 28, respectively, revealed segmental stenoses ("beading") of the basilar artery and the left middle cerebral artery and the near occlusions of both posterior cerebral arteries with thrombus adjacent to the basilar artery bifurcation. No angiographic abnormalities were observed in the extracranial carotid and renal arteries. We diagnosed the lesions as angiitic infarctions and started plasma exchange and antiplatelet therapy.