The hypothesis that hypothermia can improve functional outcome in traumatic brain injury has had several clinical failures, despite ample convincing animal data. The POLAR trial is an international, multicenter, randomized trial, examining the effects of early, sustained prophylactic hypothermia in patients with severe TBI. Hypothermia was initiated within 3 hours, sustained for at least 72 hours, and slow rewarming was initiated if intracranial pressure was controlled. The primary outcome was Glasgow Outcome Scale-extended score at 6 months. A total of 240 hypothermia and 226 normothermia patients were evaluated for the primary outcome. Favorable outcome occurred 48.8% in the hypothermia group and 49.1% in the normothermia group (95% CI, -9.4 to 8.7 percentage points). There was also no significant differences in any of the secondary outcomes. These findings support the idea that there is currently no role for initiation of hypothermia in the acute phase of TBI management.