The Turbidity and Gastroenteritis Study aims to establish and characterize the link between the turbidity level of distributed water and the number of cases of acute gastroenteritis (AGE) observed in the service area. This part of the study covers two Parisian arrondissements (the 19th and 20th), supplied by a mixture of water from the treatment plants in Ivry-sur-Seine (pumping from the Seine) and Joinville-le-Pont (pumping from the Marne). The treatment processes utilize slow sand filtration. The incidence of AGE was obtained from health insurance data between 2002 and 2007. The method involved regressing, using a generalized additive model, the incidence of AGE against the daily turbidity levels of the distributed water. In a second step, several plant operating parameters and meteorological factors were tested as risk factors for AGE. The results show that a deterioration in the turbidity of the distributed water is linked to an increased risk of AGE among children (ages 1–15) and adults (ages 16 and older). The flow rate of the water source strongly influences the relationship between turbidity and the risk of AGE, as this relationship holds true only during periods of high water flow and not during periods of low water flow. The type of optional treatment implemented at the Joinville-le-Pont plant (settling or flotation) has no significant influence on the risk of AGE. (R.A.)