BACKGROUND: S100 beta protein has been reported to be an early marker of brain injury. It is released from the glial cell and Schwann cell specifically after brain injury, and it,s serum concentration correlates with the severity of injury. The aim of this study was to measure the serum concentration of S100 beta protein during cardiac surgery and to reveal the correlation between cerebral oxygenation and S100 beta protein. METHODS: Eighteen patients undergoing cardiac surgery using cardiopulmonary bypass (CPB) were enrolled in this study. After induction of general anesthesia, the arterial-jugular venous oxygen difference (AjDO2), regional cerebral oxygen saturation (rSO2) and lactate oxygen index (LOI) were measured. They were measured after induction (T0), during CPB (T1) and at the end of CPB (T2). Serum S100beta protein was measured at T0, T2, T3 (5 hours after CPB), and T4 (24 hours after CPB) using an immunoluminometric assay. We observed correlations between rSO2, AjDO2, LOI and the S100beta protein concentration. RESULTS: Serum concentrations of S100 beta protein were 0.18 +/- 0.20, 5.72 +/- 4.25, 1.06 +/- 1.38 and 0.58 +/- 0.44 (micro gram/L) at T0, T2, T3 and T4 respectively (normal value