A pilot study was conducted to assess the potential of French National Health Insurance drug reimbursement data for the automated identification (clustering) of waterborne outbreaks. The weekly number of hospitalized cases of acute gastroenteritis (AGE) was derived from this database for the 1,543 municipalities in three predominantly rural French departments over the 2009–2012 period. A clustering method, combining two simple methods, was based on the spatial comparison of incidence rates (Method A) and on the spatial comparison of the temporal trends in these incidence rates (Method B) between the targeted municipality and the municipalities in the department. For each department, each municipality was tested, week by week, throughout the study period. Two hundred and ten weekly clusters involving more than 5 cases of GEAm were identified by this method using detection criteria derived from field investigation practices. These clusters form 193 consolidated clusters lasting 1 to 3 weeks. One in ten municipalities experienced one cluster, and fewer than two in 100 experienced multiple clusters. Extrapolation to the whole of France indicates that 1,000 clusters (representing a total of 30,000 cases of waterborne gastroenteritis) could be detected annually. The impact represented by these clusters accounts for only a small proportion of the total number of waterborne gastroenteritis cases. Rather than merely providing information on the infectious impact of water, the objective of this clustering is to stimulate field investigations, direct them toward the highest-risk situations in coordination with the implementation of Water Safety Plans, and support the monitoring of risk factors at the national level. This method appears to be improvable. A comparison with other methods must also be conducted before the final selection of the method to be implemented at the national level. (R.A.)