Diffuse Lewy body disease (DLBD) has been studied from various viewpoints and, although clinical diagnostic criteria for DLBD have been proposed, diagnosis remains difficult. DLBD has been reported to be the second most common form of dementia in the aged, following Alzheimer-type dementia. It has, however, been clinically under-diagnosed. Therefore, the search for diagnostic markers for DLBD must continue. Very recently, ‘dementia with Lewy bodies’ (DLB) was proposed as a generic term for various forms of dementia with Lewy bodies, including DLBD and similar disorders. Cortical Lewy bodies are the most important pathologic marker for diagnosis of DLBD. At present, however, the mechanism responsible for cortical Lewy body formation has yet to be disclosed.