Cerebral oxygenation level during cardiopulmonary bypass (CPB) was measured using near-infrared spectroscopy as a monitor of cerebral circulation in 30 patients. Six adult cases with thoracic aortic aneurysm were operated on using selective cerebral perfusion (SCP). CPB was established under moderate hypothermic temperature in 9 adult cases (hypothermic group, lowest blood temperature during CPB; 25 degrees C) and under normothermic temperature in 9 adult cases (normothermic group, lowest blood temperature during CPB; 34 degrees C). In congenital cases (n = 6), CPB was established under moderate hypothermic temperature (congenital group, lowest blood temperature; 25 degrees C). The oxyhemoglobin (HbO2) level showed a significant positive correlation with cerebral blood flow during SCP (r = 0.715). There was no significant correlation between SjO2 and HbO2 in the SCP group. The HbO2 levels in the hypothermic group after 30 and 60 min, from the initiation of CPB and 30 min. before the weaning of CPB were significant lower than the control level (p < 0.05). HbO2 levels in the congenital group after 0, 30 and 60 min. from the initiation of CPB and 30 min. before the weaning of CPB were significantly lower than the control level (p < 0.01). The deoxyhemoglobin (HbR) level in the hypothermic group after 30 and 60 from the initiation of CPB and 30 min. before the wearing of CPB were significantly higher than the pre level (p < 0.05). The mixed venous saturation (SvO2) in the normothermic group showed significant lower levels than those in the hypothermic group (p < 0.01). However, there was no significant difference in HbO2 levels between the two groups. In conclusion, these results suggest that near-infrared spectroscopy may be a noninvasive and useful technique for the cerebral circulation monitoring during CPB.