Although the sensitivity of syndromic ILI definitions was not high by clinical standards, our interest was to understand the proportion of influenza cases in the community being captured by the system. ED ILI may provide a more robust estimate of the burden of disease than laboratory surveillance, which captures only a subset of patients seen by a healthcare provider and who were tested. The higher specificity of the ILI definitions suggests it is best used for situational awareness during influenza outbreaks and for distinguishing influenza from other viral agents. The use of several definitions throughout the season may be most appropriate in some settings. Public health practitioners should bear in mind that syndrome performance may vary by season and year. Higher syndrome specificity among older children suggests specificity in adults should be higher than that observed for younger populations. However, the generalizability of these results to adult populations and other hospitals is uncertain and should be further studied.