During cardiac surgery, esophageal, rectal, or bladder temperature is usually monitored as an index of core temperature; however, these methods are invasive, and often inconsistently reflect the central body temperature, especially in pediatric patients. The purpose of this study was to evaluate the utility of tracheal temperature monitoring during cardiac surgery in pediatric patients. Fifteen children (ages; 8 m.-7 yr.) undergoing cardiac surgery with cardio-pulmonary bypass (CPB) were studied. Anesthesia was induced and maintained with high doses of fentanyl and intermittent doses of midazolam. After anesthetic induction, esophageal, rectal, bladder, tympanic, and forehead deep temperatures were monitored with a Core Temp Monitor (CTM-205, Terumo Co.). Simultaneously, the tracheal temperature was monitored with a specially made tracheal tube. A thermistor was attached by medical glue (Loctite Prism, Loctite Co.) at the anterior surface of a tracheal tube (Trachelon, Terumo Co.) without cuff (inside diameter 4.0-6.0 mm) where the tracheal tube tightly fits against the trachea. The inspired gas was warmed to 35 degrees C and humidified. During CPB, the blood temperature at the inlet to the patients was also recorded simultaneously. All indicated temperatures (Y) during CPB were analyzed for correlation with the blood temperature to the patients (X). The tracheal temperature had the highest correlation with the blood temperature from the patients (Y = 0.68X + 10.60, r = 0.89). There were also good correlations of the esophageal as well as bladder temperatures with blood temperature. There were no patients who suffered tracheal inflammation or laryngeal edema from the thermistor. Monitoring tracheal temperature is not only valuable for monitoring the core temperature, but also is convenient for pediatric patients in cardiac surgery.