Positron emission tomography (PET) with 18F-2-fluoro-2-deoxy-D-glucose (FDG) demonstrates a typical pattern of impairment of regional metabolic rates of glucose (rCMRGlu) in most patients with a clinical diagnosis of probable Alzheimer's disease (AD): reduction of rCMRGlu in temporo-parietal association cortex, more variably also in prefontal cortex, but relative preservation of primary visual and sensoriomotor cortex, striatum, and cerebellum. Apart from early stages, both hemispheres are affected, but pronounced asymmetries may be present. With the exception of Parkinson's disease with dementia, the complete pattern is rarely seen in other dementing conditions, which usually lead to more global, frontal or multifocal metabolic impairment. Severity of dementia is mainly correlated with temporo-parietal rCMRGlu reduction, probably irrespective of the cause of dementia, and the neuropsychological profile is related to the asymmetry of metabolic alterations. Procedures are available for assessment of the typical pattern that yield comparable results in different laboratories, and have a high accuracy for discrimination between normals and probable AD. Diagnostic accuracy is better for presenile than for senile dementia of Alzheimer type, and for moderate to severe cases than for mild dementia. A definitive judgment of the diagnostic value of FDG PET in AD is hindered by the lack of sufficient data with diagnosis confirmed at autopsy.