Cerebral hyperthermia caused by perfusate temperature greater than 37 degrees C during the rewarming phase of CPB has been linked to postoperative neurologic deficits. The purpose of this study was to determine the accuracy of the coupled temperature measurement system and the CDI 500 arterial temperature sensor. Seventeen patients undergoing CPB were divided into four groups, each with a different temperature probe coupled to the oxygenator. The coupled temperature measurement system and CDI temperature sensors were compared with an indwelling probe placed in direct contact with the arterial perfusate. Blood, bladder, room and water temperatures, arterial line pressure, blood flow, and hemoglobin were recorded while the patients were supported with CPB. The actual blood temperature was significantly higher than the coupled temperature measurement system for two of the four groups (mean = 1.61 degrees C and 0.91 degrees C, p < 0.0001). A significant positive correlation between the actual temperature and the coupled temperature measurement system error was observed for the same two groups (r = 0.44, p < 0.0001). The actual temperature was significantly higher than the CDI temperature in all patients (mean = 1.2 degrees C, p < 0.0001). The coupling mechanism on the oxygenator generates inconsistent temperature readings. The perfusionist should consider these inconsistencies when using coupled temperature measurements and may consider the use of a direct temperature measurement system. The CDI temperature error is probably the result of inadequate flow through the sensor. On the test circuit, the flow of 170 mL/min was inadequate for circuit temperature accuracy. The accuracy of the CDI temperature drastically improved when the flow-through the sensor was increased to approximately 400 mL/min. Thus, the perfusionist must ensure adequate flow through the sensor in order for the temperature mechanism to function properly. Finally, the perfusionist can prevent cerebral hyperthermia by not allowing water temperature to exceed 37 degrees C, when using a coupled temperature measurement system.