Postoperative effects of extended rewarming (ECR) after hypothermic cardiopulmonary bypass (CPB) were studied. All (n = 28) patients were rewarmed to a nasopharyngeal temperature exceeding 38 degrees C before terminating CPB. In 12 patients (control group) the rectal temperature (Tre) was 33.8 +/- 1.7 degrees C (mean +/- sd) at termination of CPB. In sixteen patients (ECR group) rewarming during CPB was continued to a Tre of 36.8 +/- 0.5 degrees C. Postoperative body temperatures, heat content, shivering, oxygen uptake, CO2 production and haemodynamic variables were measured. ECR reduced the heat gain required to complete core rewarming to 665 +/- 260 kJ, compared with 1037 +/- 374 kJ in the control group (p less than 0.01). The incidence of shivering was reduced (p less than 0.05) as well as shivering intensity and duration. In seven non-shivering ECR group patients this coincided with significantly reduced metabolic and ventilatory demands but these improvements were not valid for the group as a whole. The required ventilation temporarily during postoperative rewarming in both groups increased to 250 per cent of the basal need. Extending CPB rewarming (to at least 36 degrees C Tre) was inefficient when used as the sole measure to reduce the untoward effects of residual hypothermia during recovery after cardiac surgery with hypothermic CPB.