Studies of behaviour disorders and psychotic symptoms in patients with Alzheimer's disease have shown that the most frequently observed disorders and symptoms are: delusions, hallucinations, activity disorders and aggressive behaviour. Presence of these different symptoms have been found to correlate with each other. Their manifestations and dynamics suggest that delirium is more frequent in Alzheimer's disease than clinical diagnoses suggest. Problems inherent in the diagnosis of dementia delirium are discussed. New diagnostic criteria for severely and very severely dementia patients, developed on the basis of the author's own research and analysis of the literature, are suggested.