National studies determining the burden of gastroenteritis have defined gastroenteritis by its clinical picture, using symptoms to classify cases and non-cases. The use of different case definitions has complicated inter-country comparisons. We selected four case definitions from the literature, applied these to population data from Australia, Canada, Ireland, Malta and the United States, and evaluated how the epidemiology of illness varied. Based on the results, we developed a standard case definition. The choice of case definition impacted on the observed incidence of gastroenteritis, with a 1·5&-2·1 times difference between definitions in a given country. The proportion of cases with bloody diarrhoea, fever, and the proportion who sought medical care and submitted a stool sample also varied. The mean age of cases varied by <5 years under the four definitions. To ensure comparability of results between studies, we recommend a standard symptom-based case definition, and minimum set of results to be reported.