OBJECTIVE To compare the ability of forced-air warming and reflective insulation to maintain intraoperative normothermia. METHODS Prospective, randomized clinical trial. METHODS Operating rooms of a general hospital. METHODS 20 ASA physical status I and II patients undergoing elective total hip arthroplasty. METHODS Patients were randomly assigned to be warmed intraoperatively using forced-air or reflective insulation. Inspired gases were conditioned using a heat-and-moisture exchanger in both groups, and infused intravenous fluids were warmed to 37 degrees C. RESULTS Distal esophageal (core) temperatures decreased approximately 0.5 degrees C in both groups during the first 45 minutes of anesthesia. Subsequently, core temperatures increased slightly in the patients given forced-air warming. In contrast, core temperatures continued to decrease in patients covered with reflective insulation. After 135 minutes of anesthesia, core temperatures were 36.4 +/- 0.6 degrees C (mean +/- SD) in the forced-air group but only 35.4 +/- 0.6 degrees C in the insulated group (p < 0.01, unpaired t-test). These data indicate that forced-air warming is superior to reflective insulation. CONCLUSIONS Reflective insulation was unable to maintain intraoperative normothermia during total hip arthroplasty. Active warming, such as that provided by forced air, was required to prevent hypothermia.