Acute disseminated encephalomyelitis is an acute demyelinating disorder of the central nervous system that usually occurs in children and young adults. We report the case of an 8-year-old girl who presented to the emergency department with acutely altered mental status. Standard workup including head computed tomography, lumbar puncture, and routine chemistries was unrevealing. Magnetic resonance imaging revealed findings consistent with acute disseminated encephalomyelitis. Response to treatment with steroids was dramatic. Both the rapidity of onset and resolution of this patient's symptoms are unusual for the course of this disease.