This paper describes a proposed study to investigate the incidence of dementia in a population of over 75 year olds in Melton Mowbray, Leicestershire, England. The study population (n = 438) underwent a detailed psychiatric assessment using the Cambridge Examination for Mental Disorders of the Elderly (CAMDEX) as the second stage of a survey of the physical and mental health of the elderly begun in 1988. The CAMDEX assessment and the initial screening interview included information on the majority of potential risk factors for Alzheimer''s disease and vascular dementias. Together with measures of incidence and risk factors, the proposed study aims to assess the effect of the decline in mental function of the elderly, on carers, family and health services and to suggest where improvements in planning and delivery of services might be made. A number of problems arise, particularly relevant for studies of the elderly, due to the frequent follow-ups necessary for incidence studies and these are discussed.