Two patients with arteriovenous malformation were studied by position emission tomography, using the steady-state technique with 15O2. In the first patient, who had seizures, preceded by auditory hallucinations, an area of decreased cerebral blood flow and oxygen consumption was shown just behind the malformation. In the second patient, who had repeated attacks of right hemiparesis, an area of decreased blood flow and oxygen metabolism was shown at distance of the malformation. These areas of low flow and metabolism are most probably the result of a vascular steal phenomenon and the origin of the transient neurological symptoms in both patients.