To investigate the time relation between dementia and major affective disorders (major depression and mania). Register linkage study of the Danish Hospital Register and the Danish Psychiatric Central Research Register, to establish study cohorts of patients with dementia and control groups (osteoarthritis or diabetes) on first discharge from hospital. Follow up of cohorts was for up to 21 years. Hazard of death was allowed for by the use of competing risks models. Patients with dementia had an increased risk of being admitted to hospital for major depression or mania during the course of the illness. The incidence remained elevated throughout the rest of the patient's life. Patients with dementia have an increased risk of developing depression or mania. Proper treatment of affective disorders in patients with dementia is important in reducing suffering and costs.
