An analysis of four areas of the cerebral cortex, from pia to white matter, in presenile dementia, demonstrated a substantial and constant neuron loss of 57% in all areas. In contrast with normal ageing, this cell loss is not continuous over the whole cortex. It is maximal in the lower cortical half. The conclusion was drawn that presenile dementia is not a form of early ageing. Dementia seems to be related to a change in the interspatial arrangement of the efferent and afferent elements of the cortex. The most integrative part of the cerebral cortex (layers II and III) shows the most resistance to destruction.