The use of an ultrasonic transcranial Doppler technique for noninvasive evaluation of cerebral vasospasm is described. Middle cerebral arteries (MCA's), classified as spastic on angiography, demonstrated blood-flow velocity between 120 and 230 cm/sec. The flow velocities in these arteries had a clear inverse relationship to the diameter as measured from angiograms in 38 patients with recent subarachnoid hemorrhage. This relationship in the proximal anterior cerebral artery (ACA) was found to be more complicated to assess, due to the collateral channels in the anterior part of the circle of Willis. The authors conclude, however, that the new method of measuring vasospasm will also detect spasm in the ACA if it has a hemodynamically significant effect upon flow resistance.