We found less variation between jurisdictions in weekly ratios using the common ILI definition (mean 2%; range 1.5Á3.1%) than locally preferred syndromes (mean 4.9%; range 1.8-8.4%), and influenza epidemic signal-to-noise ratios were comparable for most jurisdictions during the study period. The findings suggest that the common syndrome improves comparability without an overall cost in terms of epidemic signal discrimination.