Factors affecting body temperature in the perioperative period were investigated in two groups of surgical patients (cholecystectomy and repair of fractured femur). Core, toe skin and thigh skin temperatures were assessed pre and postoperatively for all subjects. A control group showed a significant fall in toe skin temperature only during exposure to theatre conditions, whereas all of the surgical patients underwent falls in core temperature. The fractured femur patients sustained significantly greater reductions in core temperature than the cholecystectomy patients. Age and ambient theatre temperature were found to be the most influential factors affecting this fall in core temperature. Body composition had less of an influence than these two variables, but it was found that the patients with the greatest body fat contents had the smallest reductions in core temperature. Seven of the 31 patients returned to the ward with core temperatures below 35°C, and only one of these seven received assistance in rewarming. Four of the 31 patients developed significantly higher core temperatures than the remaining 27 during the immediate postoperative period, and these four subsequently developed respiratory tract infections. Standard monitoring of temperature on the wards during the immediate postoperative period failed to identify postoperative hypothermia or pyrexia in the majority of cases.