Regional cerebral blood flow (CBF), oxygen utilisation, fractional oxygen extraction (OER) and cerebral blood volume (CBV) were measured by positron emission tomography (PET) in 21 patients with occlusive carotid artery disease. In the same patients, measurements of cerebral CO2 reactivity were performed using the intravenous xenon-133 technique. A significant correlation was found in symptomatic hemispheres between the CBF/CBV ratio and CO2 reactivity. Four patients had significant increases in OER and this was associated with a reduction in CBF/CBV ratio implying exhaustion of haemodynamic reserve. CO2 reactivity was reduced below 1.5% mm Hg in all four cases with raised OER but only in two cases with normal OER. In patients with CO2 reactivities above 1.5% mm Hg, OER was normal in all cases. It is concluded that measurements of CO2 reactivity provide a satisfactory method for assessing cerebral haemodynamic reserve.