To provide understanding of influence of cardiopulmonary bypass (CPB) on cerebral blood flow (CBF), we investigated the effect of CPB on patients' cerebral haemodynamic parameters. Twenty-three patients were prospectively enrolled. CBF was estimated by transcranial Doppler (TCD) to measure blood velocity in the middle cerebral artery (MVMCA), preoperatively T(0) and at four postoperative times (T(1), T(2), T(3), T(4)). At times T(2), T(3) and T(4), MVMCA remained at higher levels than T(0) (P < 0.05). In the multivariate analysis PaCO(2) was independently associated to MVMCA at times T(1) and T(2) (P = 0.03, P = 0.01, respectively) and temperature was independently associated with MVMCA at time T(1) (P = 0.02). Thus, the present study showed an increase in CBF after CPB, that was correlated with raised temperature but not with decrease in haematocrit.