We have evaluated a new clinical thermometer for accuracy. It determines temperature by measuring infrared radiation given off by a warm object. It was found to be accurate both in vivo and in vitro over the temperature range of 34.0 degrees to 39.5 degrees C. The in vivo assessment was made in patients in the ICU after their return from open heart surgery. The infrared tympanic thermometer tracked the core temperature (as measured by the thermistor tip of the pulmonary artery catheter) closely, with a correlation coefficient of 0.98, and took less than 2 sec to measure. We found this new thermometer to be accurate and, in our opinion, easily usable in the clinical situation.