We have investigated patients undergoing cardiac surgery with hypothermic bypass to see if the addition of skin surface warming during systemic rewarming on bypass (heated group, n = 43) would improve perioperative thermal balance compared with conventional management without skin warming (control group, n = 43) in an open, randomized, controlled study. Intraoperative skin warming with a water mattress and forced warm air over the face, neck and shoulders attenuated the afterdrop in nasopharyngeal temperature after weaning from bypass (2.3 (1.2) degrees C and 1.3 (0.5) degrees C in the control and heated groups, respectively) (P < 0.05) and resulted in higher rectal temperature 4 h after surgery. Despite similar standard coagulation tests, heated patients had lower blood loss via the chest tubes (600 (264) ml vs 956 (448) ml in control patients) (P < 0.05) and less requirements for i.v. colloid infusion (1662 (404) ml vs 1994 (389) ml) (P < 0.05). There was a significant inverse correlation between rectal temperature on arrival in the ICU and postoperative blood loss (r = 0.57, P < 0.001). These data suggest that additional skin surface warming with a water mattress and forced warm air helped to preserve perioperative thermal balance and may contribute to reduced bleeding after cardiac surgery.