A young girl with an unremarkable medical history presented with 1 week of drowsiness, double vision, and left cranial nerve VI and VII palsies. Brain magnetic resonance imaging (MRI) revealed T2-weighted and fluid-attenuated inversion recovery changes with associated patchy enhancement in the brainstem, left frontal lobe, left cingulate gyrus, and left parietal lobe without restricted diffusion (Figure, A and B). Cervical spine imaging results were negative. Cerebral spinal fluid (CSF) study results were unremarkable (red blood cell count, 0 × 106/µL [to convert to ×1012/L, multiply by 12]; white blood cell count, 0/µL [to convert to ×109/L, multiply by 0.001]; glucose, 61 mg/dL [to convert to millimoles per liter, multiply by 0.0555]; and total protein, 19 g/dL [to convert to grams per liter, multiply by 10]), and CSF cytologic test results were negative for malignancy. Oligoclonal bands and IgG index were not measured. The results of serum cell-based aquaporin 4 antibody tests were negative. She was diagnosed with acute demyelinating encephalomyelitis (ADEM) and received 5 days of high-dose intravenous corticosteroids followed by a 1-month course of oral prednisone with improvement back to baseline in 3 weeks.