OBJECTIVE Increased transcranial Doppler velocities and regional cerebral perfusion defects have been well demonstrated in patients with subarachnoid hemorrhage, but the clinical significance of these changes has not been clearly defined, particularly in the presymptomatic stage of cerebral vasospasm. We have tested the hypothesis that a rapid, massive rise in Doppler velocity denotes progressive vasospasm by relating Doppler velocity increases to regional cerebral blood flow changes and to the subsequent clinical course. METHODS Serial transcranial Doppler sonography was performed in 121 patients; 20 of these patients were selected for blood flow mapping on the basis of rapid increases (greater than 50 cm/sec/24 hr) in blood flow velocity. Cerebral blood flow was mapped by single-photon emission computed tomography using technetium-99m hexamethylpropyleneamine oxime. RESULTS Ten of 15 patients studied before the onset of any deficit subsequently developed a focal neurological abnormality. In 14 of these 15 patients, and in a further five in which single-photon emission computed tomography was performed after the onset of a delayed neurological deficit, perfusion patterns were abnormal and correlated with sites of increased Doppler velocities. Four patients had zones of cerebral hypoperfusion but did not develop neurological deficit. CONCLUSIONS Transcranial Doppler measurements can assist in identifying patients at risk of delayed ischemic deficit. Selection of patients for regional cerebral blood flow mapping studies and for prophylactic anti-ischemic therapy may be considered on this basis.