Extensive research has been conducted in the use of mild hypothermia as a neuroprotectant in acute brain injury. However, little work has been done in the area of therapeutic hypothermia and cardiac arrest. Recently, the results of a multi-center randomized controlled clinical trial to evaluate the effect of mild systemic hypothermia on mortality and functional outcome after resuscitation from out-of-hospital cardiac arrest due to ventricular fibrillation or tachycardia were published, and demonstrated significantly improved survival and neurological outcome in the cooled patients. In this study, we will cool comatose out-of-hospital cardiac arrest patients to a target range of 33.5 o C to 33.9 o C. Cooling will continue for 24 hours and be followed by gradual rewarming to 36.0 o C over 6 to 12 hours.