We studied 15 patients undergoing cardiac surgery involving hypothermic cardiopulmonary bypass (CPB). Cerebral arteriovenous difference in oxygen content (AVDO2) was significantly less during CPB and for up to 18 h after operation compared with pre-CPB values (P < 0.05). There were no significant changes in mean jugular bulb oxyhaemoglobin saturation (SjvO2), cerebral arteriovenous difference in lactate content or lactate-oxygen index (LOI). SjVO2 and arterial carbon dioxide tension (PaCO2) (P = 0.005) were positively correlated as were AVDO2 and haemoglobin concentration (P = 0.012). AVDO2 and PaCO2 (P = 0.007) were negatively correlated as were LOI and arterial oxyhaemoglobin saturation (P = 0.037). There were no significant correlations between mean arterial pressure and any of the variables. SjVO2 and AVDO2 may require correction for changes in PaCO2 and haemoglobin concentration before relating these variables to cerebral outcome.