A significant number of patients suffer transient neuropsychological dysfunction after coronary artery bypass graft (CABG) surgery. Recent studies have implicated reduced levels of O2 supply/demand ratio during the rewarming phase of cardiopulmonary bypass (CPB). Using a near infrared spectroscopy (NIRS) system that permits continuous noninvasive monitoring of regional cerebral O2 saturation (rSO2), we investigated rSO2 during CPB. Following Institution Review Board approval and informed consent, 10 patients undergoing elective CABG were studied. Nonpulsatile CPB with a membrane oxygenator, haemodilution and alpha-state pH management was used. The NIRS system consisting of two low-power laser diode sources (780 nm and 810 nm) with a photodetector placed on the frontal cortex was used to measure continuously haemoglobin, oxyhaemoglobin and cerebral blood volume. Continuous rSO2 measurements were obtained before, during moderate hypothermia, and during the rewarming phase of CPB. A rSO2 < 50% was defined as abnormal. Onset of rSO2 < 50% was at 32 degrees C, and the highest incidence occurred during the late stages of rewarming (35-37 degrees C). On rewarming from hypothermic CPB, 70% of our patients sustained a rSO2 < 50% for an average total duration of 9.3 minutes, primarily at the end of the rewarming period. This suggests a transient global imbalance in cerebral O2 supply and demand in a large percentage of patients during the rewarming phase of CPB. However, it has not been determined how long a rSO2 < 50% can be permitted before neuropsychological dysfunction is induced. Additional studies correlating psychometric testing with rSO2 measurements should prove useful in detecting episodes of impaired cerebral oxygenation, and help define rewarming protocols.