On November 19, 2002, the Isère Departmental Directorate of Health and Social Affairs (DDASS) was alerted to a high number of cases of acute gastroenteritis (AGE) in four neighboring municipalities. The individual reporting these cases also noted that the well supplying the drinking water system for these four municipalities was flooded; consumption of water from the system was immediately suspected as a possible cause of the cases. A descriptive and analytical investigation was conducted to confirm the existence of an outbreak, describe it, test the hypothesis that consumption of tap water was a risk factor, and determine its etiology and environmental causes.