Currently, no on-line method of assessing cerebral oxygenation is sufficiently accurate to be clinically helpful. In an attempt to find a good predictor of postoperative cerebral outcome, we retrospectively studied the relationship between the redox behavior of cytochrome oxidase (cyt. ox.) during an operation and the neurological prognosis in 83 patients who underwent thoracic aortic surgery. Our data revealed three patterns of change in the redox behavior of cyt. ox. during the operation; the actual pattern exhibited by a given patient showed a highly significant correlation with the neurological prognosis (p < 0.0001). We conclude that the redox behavior of cyt. ox. during an operation is likely to be a good predictor of postoperative cerebral outcome, which implies that brain tissue oxygen sufficiency can be evaluated by near-infrared measurement of cytochrome oxidase (except for that in local regions far from the monitoring site).