For 80% of 4,748 patients with cerebrovascular insufficiency, complete four-vessel examinations were made. Although the primary purpose was localization of surgically accessible extracranial lesions, intracranial studies were also obtained in a high percentage of cases. The overall grave complication rate was 1.2%. A test of interinstitutional comparability of x-ray interpretation revealed greatest agreement when lesions compromised more than 50% of the arterial lumen.