There is conflicting evidence about the contribution of drinking water to endemic community gastroenteritis in water supplies which meet conventional microbiological standards with some studies reporting associations between drinking water and endemic disease and others finding no evidence that water is implicated in disease. This study reports the results of an ecological study investigating the effect on community gastroenteritis of chlorinating a city of over 3 million people in the mid 1970s. Prior to chlorination faecal coliforms were regularly identified in the water. Admissions for gastroenteritis and attendances to the Emergency Department of the Royal Children's Hospital, Melbourne's major children's hospital, were measured between 1974 and 1980 inclusive and the influence of chlorination on rates of gastroenteritis was examined. No statistically significant difference was found in the number of admissions or emergency department visits before and after chlorination of the water supply. The study highlights the need for caution when interpreting the relationships between drinking water and gastroenteritis. The result suggests that water was not a dominant contributor to the burden of gastrointestinal disease in the community despite faecal coliforms being present in the water supply. It indicates the need for caution when attributing significant illness to drinking water when there have been only small changes in water quality without first stringently reviewing the studies methodology and understanding their limitations.