An absolute increase of blood velocity in the arteries of brain arteriovenous aneurysms is of the greatest importance in their diagnosis by transcranial Doppler ultrasonography. If the branches of brain great vessels are involved in blood supply of arteriovenous aneurysms, there is an increase in blood velocity only in the initial portions of the great vessels up to the origin of these branches. A relative increase in brain great vasculature blood velocity is of great diagnostic value. Arteriovenous aneurysms whose blood supply is provided by the anterior cerebral artery are typified by higher blood velocity in the anterior cerebral artery than that in the middle cerebral artery, whereas the reverse is true for health.