Hypothermia has altered the course of military history. Military casualties tend to occur in epidemics, associated with cold weather combat or maneuvers, trauma, immobilization, improper clothing, exhaustion, and underlying illness. Symptoms of hypothermia begin subtly with fatigue and loss of concentration, but progress to stupor, coma, and resemble rigor mortis. Treatment of mild hypothermia is with body heat and warm clothes and fluids. Moderate and severe cases require gentle evacuation and active core rewarming method(s). Inhalation of warm (40 degrees C, 104 degrees F) humidified oxygen is safe, effective, and can be begun in the field. Recognition of risk factors and active measures can lessen the menance of cold weather for military personnel.