The progression of the prevalence of dementia in developed countries will increase the difficulties of medical and psycho-social management of demented patients and their family. The aim of this study was to estimate the prevalence of dementia among subjects aged 75 years and over. These subjects have been followed-up ten years after the baseline examination of the PAQUID study, a prospective population-based cohort study on normal and pathological aging after 65 years. A total of 1,461 subjects, aged 75 years and over, have been visited. The prevalence of dementia was estimated to be at 17.8 p.cent. About 38.5 p.cent of these demented subjects ware living in an institution, which leads to the fact that more than the two thirds of the subjects living in an institution were demented persons. Alzheimer's disease was the main etiology of dementia (79.6 p.cent). Based on the French population census, currently there would be about 769,000 demented people aged 75 years or over in France. Dementia accounts for 72 p.cent of the potential need for the French subvention for dependent persons, called Allocation Personnalisee d'Autonomie P. These results point out the importance of regarding dementia as a Public Health priority and of developing necessary measures for caring for these subjects, whether residing in their home or an institution.