A case of acute encephalopathy with selective bilateral symmetrical striatal lesions is reported. The patient was a previously healthy 4-year-old boy who became obtunded after a febrile illness and fell into a state of delirium with severe pain in the feet. He showed abnormal postures: hyperextension of the neck and upper limbs and extreme flexion of both lower limbs, and abnormal involuntary movements of the limbs: tremor, athetotic movement and right hemiballismus. Analysis of serum antibody titres suggested recent primary infection of herpes simplex type 1 (HSV-1). Cranial T2-weighted magnetic resonance imaging (MRI) demonstrated areas of high-signal intensity involving the whole basal ganglia bilaterally. He showed rapid clinical improvement after the initiation of corticosteroid therapy; complete clinical recovery was noted 3 months after the onset. Serial MRI studies demonstrated a rapid reduction of the lesions, resulting in only slight T2-hyperintense areas in both caudate nuclei. The pathogenesis of the disorder remains unknown, though an autoimmune mechanism has been speculated. The clinical and laboratory findings in this case suggested a possible role of HSV-1 in the pathomechanism of the disorder and a beneficial effect of early corticosteroid therapy.