On November 19, 2002, the Isère Departmental Health and Social Services (DDASS) was alerted to a high number of cases of gastroenteritis in the municipalities of Apprieu, Colombre, Saint-Blaise-du-Buis, and Réaumont, all of which are supplied by the same water system. A descriptive and analytical investigation was conducted to confirm the existence of an outbreak and to determine its infectious and/or environmental source.