We report the vasocapacitance of the cerebral circulation, as determined by cerebral blood flow reactivity to induced hypercapnia using fluoromethane positron emission tomography, in 32 patients with unilateral anterior circulation transient ischemic attacks. A hemodynamic subset of eight patients, defined based on exertional, positional, orthostatic, or cardiac dysrhythmic induction of symptomatology, is characterized by multiple (median, 4.5 attacks per patient), brief (median, 2.5 minutes per attack), continued episodes of hemispheric ischemia including focal limb shaking. Symptomatic middle cerebral artery flow territories show significantly lower (p less than 0.04) and more asymmetric (p = 0.036) vasodilatory responses in the hemodynamic subset. Although ipsilateral internal carotid artery occlusion is more prevalent in the hemodynamic subset, the features of age, mean arterial blood pressure, carbon dioxide values, serum glucose, serum hematocrit, and number or type of risk factors do not differ significantly between groups. These studies of vasocapacitance help validate clinical criteria for cerebral hemodynamic events with an objective physiologic measurement.