Regional cerebral blood flow (CBF), oxygen utilisation (CMRO2) and fractional oxygen extraction (OER) were measured by positron emission tomography in patients with multi-infarct dementia. A matched reduction of both CBF and CMRO2 in the majority of patients indicated that in general, reduced cerebral perfusion in this condition is appropriate for the reduced metabolic demands of a damaged brain. Only in a very small minority of patients with dementia associated with bilateral carotid artery occlusion was CBF inappropriately low and OER raised. In these exceptional cases with critical cerebral perfusion, surgical measures to increase CBF could theoretically arrest or slow down the progress of dementia.