A scintillation camera connected to auxillary equipment with off-line data processing or connected to an on-line dedicated computer system permits measurements of hemispheric and regional cerebral blood flow. Reliable flow values are obtained from regions limited in size by spatial resolution and the count rates achieved. Flow measurements obtained with the camera are able to resolve inhomogeneities of cerebral circulation in normal subjects. In a variety of clinical conditions, the localization, severity and extent of flow alterations are shown. Results of flow measurements in individual cases elucidate the pathogenesis of neurologic deficits, quantify the damage to the brain, indicate therapeutic meausures of potential value and permit an estimation of the further clinical course. With restricted spatial resolution, flow meaurements after intravenous 133Xe injection are also feasible.