BACKGROUND: Cerebral oxygen saturation during cardiopulmonary bypass (CPB) has been known to affect the postoperative neurological sequelae. In this study, we compared the oxygen saturation determined by cerebral oxymeter (INVOS 5100) or direct internal jugular vein sampling. METHODS: Thirty patients scheduled for elective coronary artery bypass graft surgery were randomly divided into two groups. Group 1 underwent surgery during hypothermia and group 2 underwent surgery during normothermia. The regional oxygen saturation (rSO2) and the juglar venous oxygen saturation (SjvO2) were measured by a spectrophotometer probe attached to the mid-forehead and by a catheter inserted into the left jugular bulb, respectively. RESULTS: In hypothermic group, the SjvO2 tended to increase during CPB but the rSO2 was significantly decreased. After CPB weaning, both the SjvO2 and the rSO2 were returned to control value. In normothermic group, the SjvO2 and the rSO2 were decreased during CPB and there was no difference between two values. No neurological sequelae were seen in both groups. CONCLUSIONS: These results suggest that the rSO2 may not reflect the SjvO2 accurately during hypothermic CPB unlikely to normothermic bypass.