Autonomic hyperactivity is a life-threatening manifestation of many disorders affecting the central or peripheral nervous systems. Sympathetic hyperactivity manifests with hypertension, tachyarrhythmias, hyperhidrosis, peripheral vasoconstriction, and hyperthermia or hypothermia. Massive sympathoexcitation can lead to intracranial hemorrhage, vasogenic brain edema, congestive heart failure, apical ballooning (Takotsubo) syndrome, and neurogenic pulmonary edema. Less commonly, autonomic hyperactivity also involves the parasympathetic system and manifests primarily with bradyarrhythmia or even syncope. Common causes of autonomic hyperactivity are head trauma, hypoxic brain injury, subarachnoid hemorrhage, autonomic dysreflexia in spinal cord injury, Guillain-Barré syndrome, and iatrogenic disorders such as neuroleptic malignant syndrome and serotonin syndrome. Autonomic hyperactivity in the setting of cognitive, psychiatric or sleep disturbances suggests alcohol (or other antidepressant drugs) withdrawal, fatal insomnia, or limbic encephalitis. Patients with autonomic hyperactivity should be managed in an intensive care unit as they require continuous monitoring of cardiac rhythm, blood pressure, respiration, and fluid balance.