Capability and limitation of transcranial Doppler (TCD) in assessing cerebral collateral flow were evaluated in 25 patients. Changes in velocities and flow directions in basal cerebral arteries were assessed with compressive maneuvers of the common carotid artery and plotted on the three dimensional zero-float computer graphic display. Preoperative TCD examinations were combined with angiographic examinations. 16 of the 25 patients who required temporary or permanent occlusion of the internal carotid artery for treatment of their cerebrovascular and neoplastic lesions were evaluated for the feasibility of carotid clamp or ligation. In 13 of the 16 patients whose neurological function and intracranial vessel flow velocities were maintained adequately, temporary or permanent ligation of the carotid artery could safely be undertaken. In 3 of the 16 patients whose flow velocities decreased to zero, EC-IC bypass was performed prior to the carotid occlusion. TCD findings demonstrated angiographic findings of cross-filling. Collateral capacities of the extracranial-intracranial bypass were examined in the remgining 9 of the 25 patients including 6 patients who underwent vein graft (VG) bypass and 3 patients who underwent STA-MCA anastomosis. Bypass compression tests demonstrated the degree of contribution of the bypass to the cerebral circulation, with the VG bypass surpassing the efficiency of the STA-MCA bypass. In conclusion, TCD provides a better understanding of the hemodynamics of circulation in the brain prior to surgery for cerebrovascular and neoplastic diseases.