BACKGROUND Hypothermia is experienced by 60% to 90% of adult patients after surgery. The detrimental physiological consequences of prolonged hypothermia are a significant risk for cardiac surgical patients. OBJECTIVE To compare the effect of a warmed convective-air blanket with that of a warmed circulating-water blanket on the rates of increase in skin and core temperatures and on total rewarming time in patients with hypothermia after cardiac surgery. METHODS A quasi-experimental, repeated-measures design was used to study rewarming in 76 adult patients who were hypothermic after cardiac surgery. Subjects were randomized to two groups: 40 were warmed with a convective-air blanket; 36, with a circulating-water blanket. Skin and core temperatures were recorded every 15 minutes until the subject's pulmonary artery temperature reached 37 degrees C. Total time required for rewarming was the period between time of placement of the blanket and time of removal. The effects of 13 other variables on the time required for rewarming were also determined. RESULTS Both skin and core temperatures increased more rapidly in patients treated with the warm circulating-water blanket than in those treated with the convective-air blanket. The mean time required for rewarming was 45 minutes shorter in the group treated with the circulating-water blanket. The patient's age, volume of i.v. fluids received, length of anesthesia, starting core temperature, and treatment method had significant effects on the time required for rewarming. CONCLUSIONS The data suggest that rewarming with a circulating-water blanket produces normothermia more rapidly than rewarming with a warm convective-air blanket in adult patients who are hypothermic after cardiac surgery.