We compared data describing the proportion of subjects testing positive each week for influenza with data from CDC's ILI surveillance system and data from Google Flu Trends. A prior analysis compared Google Flu Trends data to US Census Region ILI data and demonstrated a strong correlation during the 2007-08 influenza season (R = 0.97) [2] . The correlation between Google Flu Trends data and national influenza test data was lower (R = 0.72) when assessed over five influenza seasons beginning in 2003. In terms of coefficients of determination (R 2 ), 88% of the variance is shared between Google Flu Trends and CDC ILI Surveillance, while only 51% of the variance is shared between Google Flu Trends and surveillance for laboratory-confirmed influenza. From September 2003 through May 2008, CDC ILI surveillance was more closely correlated with CDC Virus Surveillance (R = 0.85). Furthermore, the sensitivity analysis demonstrated that the Google Flu Trends correlation with the reference standard was more influenced by outlier observations than was CDC ILI Surveillance data.