The first reference in the literature to the existence of senile plaques was made as early as 1892,1 but the origin and development of such plaques are still a matter of controversy. No less than twenty different concepts regarding their origin have appeared in the literature. Senile plaques are a characteristic lesion in the brains of persons who during life had suffered from advanced old age, senile dementia or Alzheimer's disease. It has been occasionally claimed that these plaques have been encountered in rare instances in the presence of a few neurologic diseases not associated with the aforementioned conditions. Senile plaques may be described2 as small, spherical, thickened, argyrophil bodies scattered throughout the cerebral cortex (fig. 1a). They are not uniform in structure, but consist of various components which are different in type and situation. Senile plaques vary in number and size, and their cellular elements may