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 six Parisian arrondissements (1st to 5th and 7th), supplied by a mix of water from the southern part of the Île-de-France region. The very high water quality allowed the operator to use only simple chlorination for disinfection. AGE incidence data were obtained from health insurance records between 2002 and 2007. The method involved regressing, using a generalized additive model, the incidence of AGE against daily turbidity levels of the distributed water. In a second step, several plant operating parameters or meteorological factors were tested as risk factors for AGE. The results show that a deterioration in the turbidity of distributed water, between 0.1 and 0.2 NFU, is associated with a 10.7% increase in the risk of AGE among children under 16 and a 3.7% increase among adults. The variable representing the turbidity level of the distributed water appears to be the best predictor of the risk of AGE. Neither the composition of the water blend used nor the total flow rate produced has a significant influence on the risk of AGE. (R.A.)