A 4-YEAR-OLD boy was lethargic and grinding his teeth. He had an upper respiratory tract infection 2 weeks earlier. His medical history was unremarkable. On physical examination, his temperature was 36.6°C; heart rate, 120 beats per minute; respiratory rate, 18 beats per minute; and blood pressure, 94/59 mm Hg. Findings from neurological examination showed his pupils dilated but equally reactive (6-4 mm). He was irritable and lethargic with normal findings from sensory examination and deep tendon reflexes. Results of a lumber puncture included clear colorless fluid under normal pressure with no red blood cell count; total white blood cell count, 10.0 × 109/L (10% neutrophils, 90% lymphocytes) (manual differential on white blood cell count in cerebrospinal fluid); glucose concentration, 3.8 mmol/L (68 mg/dL); protein level, 290 mg/L, and negative findings from Gram stain. Magnetic resonance imaging showed multiple focal areas of signal hyperintensity within the periventricular white matter, corpus callosum, internal capsule, basal ganglia, midbrain, and pons (Figure 1, Figure 2, and Figure 3). He showed positive findings from a titer for Herpes simplex virus type I (IgM). Testing for visual-evoked response and brainstem auditory–evoked response showed abnormality bilaterally and delay on the left, respectively.