A clinical tympanic thermometer was evaluated. In vitro it was found to be accurate over the temperature range 28 degrees C to 40 degrees C. Clinical assessment was made during the cooling and rewarming phases of patients undergoing surgery with the aid of cardiopulmonary bypass and compared with the results obtained with an Ellab thermocouple thermometer. It was also used in patients in the intensive care unit in whom core temperature was being measured using the thermistor incorporated in a pulmonary artery catheter. The infrared thermometer was found to be accurate, rapidly responding and easy to use.