Recent outbreaks have demonstrated that serious infectious gastrointestinal illness related to drinking water supplies remains a problem in the United States. The magnitude is unknown, but children, the elderly, and immunocompromised individuals are considered at highest risk. We examined the association between daily measures of drinking water turbidity and both emergency visits and admissions to Children's Hospital of Philadelphia for gastrointestinal illness, controlling for time trends, seasonal patterns, and temperature. We found that an interquartile range increase in turbidity levels in Philadelphia drinking water was associated with a 9.9% increase [95% confidence limits (CL) = 2.9%, 17.3%] in gastrointestinal emergency visits for children age 3 years and older 4 days later. For children age 2 years and younger, an association was found with a lag of 10 days (5.9% increase; 95% CL = 0.2, 12.0). For admissions, a similar pattern was seen. For children over 2 years old, an increase of 31.1% (95% CL = 10.8%, 55%) was seen with a lag of 5-6 days. For younger children, an increase of 13.1% (95% CL = 3.0, 24.3) was seen 13 days later. This association occurred in a filtered water supply in compliance with current federal standards.