Heat balance after cardiac surgery with hypothermic cardiopulmonary bypass (CPB) was studied in 156 patients. In spite of rewarming during CPB there was residual hypothermia at the end of operation. This heat deficit could not be prevented by intraoperative use of a heating mattress at 38 degrees C and/or heated (39 degrees C) humidified inspired gases. Postoperatively, in four groups of patients, the core and finger skin temperatures were recorded and the mean skin and mean body temperatures and heat balance were calculated. Heating of humidified inspired gases (n = 22) gave little improvement in the time course of the temperatures and heat balance as compared with that in a control group with no external warming postoperatively (n = 49). A thermal ceiling (a low-temperature radiator suspended above the bed and providing radiant heat (n = 35] significantly increased the measured temperatures, which were restored to normal earlier than those in the controls. Also, with this radiant heat postoperative shivering was almost abolished. With a combination of radiant heat and heated, humidified gases (n = 50), the postoperative heat balance was improved somewhat further. For patients treated with radiant heat, postoperative rewarming was accomplished in a shorter time and almost without active endogenous muscular thermogenesis, as was evident by the great reduction in postoperative shivering.