In two elderly patients with frontal lobe dementia and in two others with progressive aphasia an inverse relationship between the severity of beta protein deposition and the principal pathology of these disorders was noted. Deposition of beta protein occurred only in areas of cortex where functional (viable) neurones were still present and was absent where neuronal decimation had taken place. Such findings suggest that the presence of functional neurones is necessary for beta protein deposition to occur and, therefore, that neurones may be the source of the amyloid protein that is deposited within brain parenchyma not only in these disorders but also in other conditions, particularly Alzheimer's disease.