Hypothermia in trauma patients is generally considered an ominous sign, although the actual temperature at which hypothermia affects survival is ill defined. In this study, the impact of body core hypothermia on outcome in 71 adult trauma patients with Injury Severity Scores (ISS) greater than or equal to 25 was analyzed. Forty-two per cent of the patients had a core temperature (Tc) below 34 degrees C, 23% below 33 degrees C, and 13% below 32 degrees C. The mortality of hypothermia patients was consistently greater than those who remained warm, regardless of index core temperature. Mortality if Tc less than 34 degrees C = 40%, less than 33 degrees C = 69%, less than 32 degrees C = 100%, whereas mortality if Tc greater than or equal to 34 degrees C = 7%, and greater than or equal to 32 degrees C = 10%. Mortality and the incidence of hypothermia increased with higher ISS, massive fluid resuscitation, and the presence of shock. Within each subgroup (i.e., greater ISS, massive fluid administration, shock) the mortality of hypothermic patients was significantly higher than those who remained warm. No patient whose core temperature fell below 32 degrees C survived.