['To evaluate the relationship between cerebral oxygen saturation and neuropsychological dysfunction after cardiac surgery.', 'Prospective and observational study.', 'Operating room and cardiac floor of a university hospital.', 'One hundred one patients undergoing elective cardiac surgery with cardiopulmonary bypass', 'Bilateral noninvasive cerebral oxygen saturations were monitored over the forehead. The anesthetic and surgical techniques were performed as usual, and no interventions were attempted based on the monitor. Neuropsychological outcome was assessed by the Mini-Mental State Examination (MMSE) and the antisaccadic eye movement test (ASEM).', 'Preoperative baseline values of cerebral oxygen saturation (rSO(2)) were 58.6% +/- 10.2%. Patients with the nadir rSO(2) <35% had significantly higher incidences of postoperative ASEM and MMSE impairments than those with rSO(2) always above 35% (44% and 33% v 12% and 9%, respectively). Patients with areas of rSO(2) <40% for more than 10 minutes . % presented with a significantly higher incidence of postoperative ASEM and MMSE impairments than those with areas of rSO(2) <40% for less than 10 minutes . % (42% and 32% v 13% and 10%, respectively). Patients with postoperative ASEM or MMSE impairment had significantly lower nadir rSO(2) and significantly larger areas of rSO(2) <40%, <45%, and <50% than those with normal postoperative neuropsychological outcome. However, multivariate logistic regression analysis showed that areas of rSO(2) <40% were the only predictor for both postoperative ASEM and MMSE impairments.'] Is cerebral oxygen desaturation associated with early postoperative neuropsychological dysfunction in patients undergoing cardiac surgery?