A 19-year-old boy was admitted to our hospital because of impaired consciousness after experiencing fever and headaches for 7 days. The patient was in a deep coma or generalized convulsions, with retrocollis and decorticate posture. He never regained consciousness, remaining in an apallic state. The initial CSF pressure was 290 mm H2O, CSF cell count 18/3 mm3, protein 75 mg/100 ml, and sugar 74 mg/100 ml. There was no significant elevation in the viral titer in the serum or CSF. Plain CT on admission showed marked brain edema. MRI three months thereafter demonstrated symmetrical high intensity in the thalamus, the putamen and the cerebellum on both T1- and T2-weighted images, probably representing petechial bleeding. The lesions in the medial portion of the occipital lobes showed iso intensity on T1-weighted images and high intensity on T2-weighted images. These lesions were consistent with infarction of both posterior cerebral arteries, which may have been due to severe brain edema. The clinical and MRI findings were similar to those of patients with acute encephalopathy with low-density areas in the thalamus or bilateral striatal necrosis in childhood. To the best of our knowledge, there have been no reports of cases like ours in adults.