Positron emission tomography (PET) is the only method that measures concomitantly in the same cerebral region the main haemodynamic (blood flow, blood volume) and metabolic (oxygen and glucose consumption) parameters, thereby providing essential data concerning the supply of substrates by circulation and their principally neuronal/synaptic use by the cells. In "healthy" (i.e. normal) ageing, there is a progressive decrease of oxygen and glucose consumption--with subsequent reduction of cerebral blood flow (CBF)--which predominates in the neocortex and suggests a dysfunction of neuronal circuits perhaps responsible for some age-related changes in cognitive functions. The metabolic depression in the neocortex seems to be accentuated during the more "common" ageing which may include cerebrovascular risk factors, lesions of the white matter in varying numbers and/or degradation of cognitive functions. Strategically located brain lesions of vascular origin (e.g. damaging of the thalamic nuclei), may produce a thalamo-cortical disconnection which induces a diffuse cortical hypometabolism associated with severe cognitive disorders. In degenerative dementias (particularly Alzheimer's disease), which include cortical and subcortical neuronal lesions (e.g. those involving the cholinergic system), there is a hypometabolism preferentially affecting the associative cortex. The cumulative effects on cortical functioning of multiple vascular lesions, superimposed on degenerative neuron loss, might account for age-related cognitive disorders in "normal" ageing as well as in the so-called "vascular" or "mixed" dementias.