Reversed vertebral blood flow distal to a subclavian obstruction is not uncommon and rarely leads to stroke. A small subgroup of these patients have obstruction in other portions of the extracranial or intracranial circulation, however, and cerebrovascular symptoms are induced by arm exercise, which may decrease regional cerebral blood flow-at times to critical levels-indicating a true "steal" syndrome. We evaluated six patients with symptomatic subclavian steal syndrome using stable xenon with computed tomography cerebral blood flow mapping. A decrease in flow from 13% to 90% in one or more regional vascular territories was found after arm exercise. Patients with a true "steal" syndrome may be at higher risk for stroke. Measuring regional cerebral blood flow may be a means of detecting patients who have a critical loss of flow reserves and who will be symptomatically improved by cerebral revascularization.