No precise intra-operative monitoring of venous blood flow has yet been developed for neurosurgical procedures. Preservation of the bridging veins is an important issue in order to minimize complications due to disturbances of venous blood flow during an interhemispheric approach. In the present study, venous blood flow velocity was measured in 11 anterior circulation aneurysms. The measurement was performed before brain retraction, during brain retraction and after completion of the procedure without brain retraction. In 6 cases, a bridging vein was dissected completely free from the dura up to its entry into the superior sagittal sinus (SSS group). In 5 cases, dura was cut parallel to the bridging vein so that the vein remained covered by the dura (dura group). Venous blood flow velocity was measured by microvascular Doppler sonography. The results show that venous blood flow can be better preserved during the retraction when the bridging vein remains covered with the dura than if the vein is dissected completely free from the dura.