Recently, bladder temperature (BT) monitoring is employed instead of rectal temperature because it is possible to keep the probe clean. The relationship between BT and core temperature is different in patients with steady state compared with those with rapid changes in temperatures. This study evaluated BT compared with nasopharyngeal temperature (NPT) reflecting brain temperature during cardiac surgery with induced hypothermia using cardiopulmonary bypass. During the steady state, after induction of anesthesia and immediately before cooling or rewarming, BT was equivalent to NPT independent of urine flow rate. In rapid cooling or rewarming phase of cardiopulmonary bypass, BT was not equivalent to NPT. BT preceded NPT in case of a very high urine flow rate, and with a lower urine flow rate delayed BT lagged behind NPT. During rapid changes in core temperature during cardiopulmonary bypass, the difference of BT to NPT depends on urine flow rate.