We developed a new technique, extracorporeal venovenous rewarming (EVR), to rewarm hypothermic patients in the intensive care unit or operating room. We compared this method with the active external (standard) techniques of warming blankets; heated ventilator circuits, intravenous fluids, and gastric and peritoneal lavage; and cardiopulmonary bypass. The EVR technique warmed patients' blood or additional blood products and crystalloids to 40°C at 150–400 mL/min and allowed survival from a core temperature of 31.1°C after massive injury. The EVR technique rewarmed patients more rapidly than standard techniques and may be most appropriate in patients with multisystem trauma when rapid correction of hypothermia-related hypovolemia, coagulopathy, and arrhythmia is necessary. Cardiopulmonary bypass is required in severely hypothermic patients with cardiac arrest. Standard techniques can be used when these immediately life-threatening conditions are not present.