Due to the ageing of the French population, the demand for social intervention in favour of elderly persons is increasing. Since 2003, as the country’s major decentralised social action operators, the departments are in charge of responding to this demand. In a tight budget context, they must compare their respective situations according to other criteria than readily available information such as population size. New comparative data items are crucially needed. A joint study programme involving departments and national entities is currently conducted to build an information system allowing them to perform sophisticated comparisons. The author presents an analysis based on indicators designed in this framework. As regards dependency related to old age, similarities appear between departments both in terms of local demands and of local housing supply. The general classification applied to departments cannot be reproduced on a finer geographical scale. It can, however, help interpret internal disparities within a given departmental territory, as illustrated by the Loire-Atlantique case presented in the article. Local experiments may, in turn, enhance national research. Initial results yielded by this information system under construction call for further analyses.