It is important to reduce or prevent heat loss during anaesthesia, especially in patients with restricted cardiopulmonary reserves. To test a specially developed esophageal thermal tube (GK-esophageal thermal tube) for this purpose, 33 patients were randomly divided into two groups: Group A were given heat transferred to the central core during operation, using the GK-tube with circulating 41.7 degrees C warm water. Group B received no active warming. All patients were scheduled for major abdominal operation. In both groups there was a temperature fall in the induction phase. In Group B the temperature continued to fall slowly during operation, resulting in a median end-temperature of 34.9 degrees C. In Group A the temperature rose slowly after induction of heat via the tube, resulting in a median end-temperature of 36.8 degrees C in this actively warmed group. The temperature difference is significant (P less than 0.001). The median operating time was 3h 30 min in both groups. After 2 h of anaesthesia the median temperature in Group A was 36.1 degrees C and in Group B 35.0 degrees C. This difference is also significant (P less than 0.001). The described method was easy to use and without complications. We recommend this method to prevent peroperative hypothermia in all patients suspected to have limited cardiopulmonary reserves. The possible hazards and how to avoid these are described.