The results of this common ILI assessment suggest that disparate local systems can adopt a harmonized syndrome definition allowing for meaningful comparisons and national aggregation while maintaining the ability to use local systems and definitions. The common ILI syndrome provided more directly comparable time-series, both during baseline periods and epidemics. Use of the common syndrome did not have an overall or systematic cost in terms of epidemic signal discrimination. Where the signal-to-noise ratio was not improved, differences were usually minimal. Also, the use of the common syndrome did not restrict the use of the locally defined syndromes for local detection. This collaborative pilot was useful in synthesizing local experience in the creation of a nationally harmonized ILI syndrome definition.