The mixed venous oxygen saturation (SV(-)O2) was continuously monitored during hepatectomy including the resection and reconstruction of great vessels in five patients. During the artificial bypass, the inadequacy of venous return due to failed bypass and/or hemorrhage was detected by the monitoring of SV(-)O2. Low cardiac function and the effect of a vasodilator were also confirmed by this monitoring. The conventional monitoring, such as blood pressure and heart rate, did not always reflect the acute changes in patients' hemodynamics. The monitoring of SV(-)O2 is useful in the anesthetic management during the surgeries which are accompanied with acute hemodynamic changes.