Five principles of determining the case definition of foodborne illness were put forward as follows by studying the concrete practice at home and abroad. The data of epidemiologic, clinical, environmental and laboratory investigations must be analyzed comprehensively in order to define the key element and content of case definition, giving priority to objective symptoms and laboratory results. The cases whose symptoms were only subjective must be excluded. The initial and final case definitions should be determined especially when the troublemaking location is temporarily obscured. Once the troublemaking location is confirmed, the cases who did not ingest food in this location must also be excluded. A scientific explanation must be given to each of the symptomatic people, including the ones that will be excluded.=