In the Nantes metropolitan area, more than 400,000 people receive water pumped from the Loire River. This water is treated at the La Roche plant through a comprehensive process of clarification and disinfection. The quality of the distributed water met European standards. This study aimed to establish and characterize the relationship between the turbidity levels of the treated water and the number of cases of acute gastroenteritis (AGE) observed in the Nantes metropolitan area during the 2002–2007 period. The daily incidence of gastroenteritis was obtained from health insurance data. The method involved regressing, using a generalized additive model, the incidence of gastroenteritis against the daily turbidity levels of the distributed water. In a second step, the role of certain plant operating parameters or meteorological factors on the incidence of AGE was tested. The results show that a slight deterioration in water quality for the turbidity parameter, with a shift from 0.05 to 0.06 NTU, is associated with a 4% increase in the risk of developing GEA among children in the served area. Among adults, this risk is estimated at 2%. The risk appears higher during periods of high or low water levels and is reduced during periods of average flow in the Loire River. The daily discharge also influences the level of endemic risk of GEA. The reliability of the observed risk is based on several statistical criteria, such as the significance of the association, its robustness, and its reproducibility in adults. The biological plausibility of the identified association is reinforced by its consistency with data from the literature. (R.A.)