On May 26, 2000, bacterial contamination was detected in the water supply of a section of downtown Strasbourg after a major 60-centimeter-diameter water main had been put back into service the previous day following several weeks of construction work. Instructions not to consume the water in a section of Strasbourg with approximately 60,000 residents were issued on the evening news on Saturday, May 27, at 7 p.m. On Monday, May 29, the 15 Center (the S.A.M.U. emergency hotline) reported to the Bas-Rhin DDASS an increase in calls regarding gastroenteritis over the weekend. The DDASS referred the matter to the CIREI (Interregional Epidemiology Unit) to investigate a possible link between this apparent increase in cases of gastroenteritis and contamination of the municipal water supply. This article describes the materials and methods used in this epidemiological investigation, as well as its results. In this study, only tap water consumption appears to be linked to the occurrence of gastroenteritis, and a strong association between the two is demonstrated. A dose-response relationship is evident in both the matched and unmatched analyses.