Transcranial Doppler (TCD) insonation permits quantitative nonivasive evaluation of intracerebral arterial velocity. With the use of 2 MHz Doppler through a transtemporal approach, middle cerebral artery blood velocity (MCA-V, centimeters per second) and major collaterals were measured in 96 patients, including 15 normal control subjects, 66 patients with extracranial cerebrovascular disease (ECCVD), and 15 patients with other medical problems without ECCVD. MCA-V was higher in control subjects (62.7 ± 15.1) than in patients with ECCVD (45.0 ± 16.3, p < 0.05). There was no significant inverse correlation between MCA-V and the degree of internal carotid artery stenosis present by duplex examination. Twenty-three patients monitored during carotid endarterectomy had a mean MCA-V under anesthesia of 37.0 ± 16.9, which decreased to 22.4 ± 14.8 during cross-clamping (p < 0.01). MCA-V during cross-clamping correlated directly with stump pressure (R = 0.87) and was higher when major collaterals were identified before operation by TCD than when none were seen (31.7 ± 9.5 vs. 8.8 ± 8.5, p < 0.01). Shunt function was verified in all 11 patients shunted. Electroencephalographic changes occurred in four patients with an MCA-V of 14.7 ± 8.5 compared with an average of 24.1 ± 15.5 for patients with normal electroencephalograms. MCA-V increased from 46.6 ± 21.2 before operation to 61.0 ± 22.4 after carotid endarterectomy (p < 0.05). TCD can quantitate intracerebral blood flow in specific vessels, collateralization, the degree of cerebral ischemia caused by ECCVD, intraoperative changes in MCA-V that correlate with stump pressure and the electroencephalogram, shunt function, and the increases in cerebral blood flow resulting from carotid surgery. It is a new and important tool.