In the last decades, in Italy there has been a wide diffusion of electronic databases to record administrative information of relevant interest for the National Health System. Nowadays there are several administrative electronic databases maintained by the National Health System, along with other health databases such as registries for specific diseases Such a recording system can be used to conduct observational studies, if appropriately administrated. One of the specific objectives of ARCHES project is to take a census of health electronic databases existing in Italy. The census is aimed to show the main characteristics of health electronic databases having regional population as reference population, giving detailed information about their structural peculiarities, accessibility, completeness, accuracy, geographical variability and the potentialities of their use in epidemiology.