The article presents diagnostic criteria for dementia associated with diffuse Lewy body disease (DLBD) elaborated by the group of investigators from Nottingham (Byrne et al. 1991). These criteria allow to make the diagnosis of "probable" or "possible" dementia associated with DLBD. With certainty this form of dementia can only be diagnosed by neuropathological examination which reveals diffuse cortical Lewy bodies. At present there is no agreement whether DLBD is a variant of Alzheimer's disease or a separate nosological entity--the second commonest cause of dementia. The main clinical feature of DLBD is coexistence of dementia and symptoms of parkinsonian syndrome. The other important feature which differentiates between DLBD and other forms of dementia is a very considerable early fluctuation of cognitive state. Psychotic symptoms in the course of DLBD--visual and auditory hallucinations, delusions and depression--are common. At present treatment of DLBD is unknown. Treatment of psychotic symptoms is difficult because of the presence of parkinsonism.