Hypothermia, a reduction in the body's core temperature to <95.0 degrees F (<35.0 degrees C), is a preventable medical emergency usually caused by prolonged exposure to cold temperatures without adequate protective clothing. Warning signs and symptoms of hypothermia include lethargy, weakness and loss of coordination, confusion, uncontrollable shivering, and reduced respiratory or heart rate. Common risk factors are advanced age, substance abuse, altered mental status, and increased contact with substances that promote heat loss, such as water. This report describes three hypothermia-related deaths that occurred in the United States during 2003-2004, summarizes hypothermia-related mortality during 1979-2002, describes risk factors for and symptoms of hypothermia, and reviews measures to prevent hypothermia-related injury and death. Public health strategies tailored to persons at increased risk for exposure to excessive cold might help reduce hypothermia-related morbidity and mortality.