S-100beta is a specific astroglial protein whose serum level increases after cerebral injury. The purpose of this study was to investigate the correlation between elevated levels of S-100beta and the neurocognitive outcome after cardiac surgery. Fifty consecutive patients undergoing elective coronary artery bypass grafting were studied. Serum S-100beta levels were measured on induction of anaesthesia, at the 15 (th) minute, at skin closure and on the 1 (st) postoperative day. Neurocognitive outcome was evaluated by STAI-T and Zung tests preoperatively and by Mini-mental state examination every postoperative day until discharge. Neurocognitive tests and S-100beta levels were correlated within the scope of risk factors by Pearson correlation. Serum S-100beta was not detected preoperatively. Peak serum S-100beta levels were reached at skin closure in 36 of 50 patients (72%). In 24 hours, serum S-100beta disappeared in 25 patients but was still elevated in 11 (22%). A highly significant correlation was demonstrated between the duration of CPB and peak serum S-100beta levels (r= 0.91). There was a weak correlation between age and peak S-100beta levels (r= 0.62). Nine patients (18%) had a positive MMSE test which correlated well with persistent high serum S-100 levels (r= 0.98). Serum S-100beta is a promising early biochemical marker for cerebral injury following cardiac surgery within a good correlation with the CPB time, age and especially with neurocognitive tests.