For decades, cooling the body below the normal physiologic temperature has been used as a therapeutic tool. Hypothermia is used most often during cardiac surgery with cardiopulmonary bypass, as a means of protecting the brain from ischemic injury. Hypothermia is also used during some neurosurgical procedures and is being investigated as a treatment for ischemic stroke and traumatic brain injury. In this issue of the Journal, two groups of investigators report on the use of therapeutic hypothermia to prevent neurologic injury in comatose survivors of cardiac arrest.Out-of-hospital cardiac arrest claims some 225,000 lives each year in the United States . . .