Power spectrum analysis of 4-channel EEGs was performed during cooling and rewarming using cardiopulmonary bypass. During rewarming, linear correlations of temperature with the total power and with peak power frequency of the high-frequency band were observed in a significant number of cases (85%,P< 0.0001 and 76%,P< 0.002, respectively). The magnitude of these changes were 1,215 μV2/°C (±150 [SEM]) and 0.39 Hz/°C (± 0.04 [SEM]). Two other descriptors of the EEG power spectrum (the spectral edge and average frequencies) did not correlate with the temperature changes in a significant number of cases. Changes during cooling followed a similar trend but were more variable, presumbaly because of other physiologic changes associated with the start of bypass. Knowledge of the relationship of the EEG to temperature should permit distinguishing EEG changes secondary to hypothermia from those caused by acute hypoxia.