Heat loss during anesthesia and operation and subsequent hypothermia will increase the postoperative oxygen demand and may endanger patients with restricted cardiopulmonary reserves. Forty patients scheduled for intra-abdominal aortic surgery and 40 patients scheduled for peripheral vascular surgery on the lower limbs were investigated using a warming blanket, humidified heated inspired anesthetic gases at 37-40 degrees C, or both these methods together. A fourth group of patients received no active warming. A warming blanket used alone gave no protection against hypothermia when compared with no active warming. In the abdominal surgical group, there was a steady fall in temperature throughout the operation if no warming method was employed. In this group the use of humidified, heated inspired gases was significantly better than no treatment after 2 h of anesthesia (P less than 0.05). The combination of humidified and heated inspired gases and a warming blanket gave significantly better heat preservation after 40 min (P less than 0.05). Patients undergoing peripheral vascular surgery had similar but smaller drops in temperature with the different types of warming procedures employed. The differences in temperature between the intra-abdominal and extra-abdominal operations were statistically significant after 3 h (P less than 0.05).