Measurements of flow velocity in the basal cerebral arteries can be obtained non-invasively through the intact skull using 2 MHz pulsed Doppler ultrasound. We investigated 47 patients with subarachnoid hemorrhage and compared flow velocity with angiography. Flow velocity in the middle and posterior cerebral arteries (MCA and PCA) was inversely related to the lumen diameter. Further clues to diagnose MCA spasm were obtained from the ratio of MCA flow velocity divided by the velocity in the distal extracranial internal carotid artery. In the individual patient, the two anterior cerebral arteries should be considered together, since this communicating system often has unique possibilities for collateral flow. Recordings from large MCA aneurysms were obtained before angiography in 4 patients.