NABISH study personnel will induce hypothermia to 35˚C in the ED in patients with suspected severe head injury who reach the ED within 2 hours of injury if cooling has not already begun. Patients who meet entry criteria (based upon past studies) for moderate hypothermia for 48 hours will then be cooled to 33˚C and then gradually rewarmed after 48 hours at 33˚C. Patients initially cooled to 35˚C within 2 hours of injury who prove during trauma evaluation to have exclusion criteria for 48 hours of moderate hypothermia will be warmed from 35˚C to 37˚C.