Introduction. Since 2004, the French Institute for Public Health Surveillance (Institut de veille sanitaire) has implemented a non-specific surveillance system called SurSAUD®, which includes data on the daily activity of emergency departments (EDs), to detect and describe expected or unexpected health trends. Our objective was to describe the catchment areas of EDs in Midi-Pyrénées and neighboring departments in 2008 to identify: (1) the source population to which ED surveillance data refer, and (2) the EDs from which to collect data during an unexpected health event in a given area. Method. Individual data on daily visits to 66 emergency departments in the study area were analyzed by stratifying by emergency department, area of residence, and age. To answer question 1, we calculated, for each emergency department, the proportion of visits originating from the patients’ different areas of residence. To answer question 2, we identified the most frequently visited emergency departments for each area. Maps representing these indicators were produced using a geographic information system (GIS). Results. In 2008, 1,084,978 visits were recorded in the 66 emergency departments (EDs) in the study area, representing an average of 16,439 visits per ED (range: 786 to 79,303). The most frequently visited SU accounted for 60.4% of visits within the study area (range: 6.6% to 100.0%). Recruitment areas may span multiple departments or even regions. Discussion/Conclusion. The results of such a spatial analysis could provide valuable support for non-specific surveillance. By enabling the identification of source populations for routinely collected data and the SU where to obtain data concerning a given territory, they could improve the responsiveness of monitoring expected or unexpected health phenomena (epidemics) and the assessment of the health impact of unexpected events. The analysis will soon be expanded to include medical diagnoses. (R.A.)