Methods National and regional estimates of the weekly percentage of persons seeking health care in the United States with ILI were obtained from Google Flu Trends on June 11, 2009 (http://www. google.org/about/flutrends/us-historic.txt). The remaining surveillance data were obtained from CDC. These data were from two separate surveillance networks: Outpatient Influenza-like Illness Surveillance Network (CDC ILI Surveillance) and the US Influenza Virologic Surveillance System (CDC Virus Surveillance). CDC ILI Surveillance consists of a network of health care providers who record the weekly proportion of patients who present with non-specific signs and symptoms that meet a case definition of influenza-like illness [10] . CDC Virus Surveillance consists of about 140 laboratories located throughout the United States that report the weekly total specimens tested and laboratory tests positive for influenza virus [10] . This is the only US surveillance system that provides national and regional data of laboratory-confirmed influenza virus infection. CDC Virus Surveillance is used in CDC statistical models in the estimation of influenza-associated morbidity and mortality [11, 12, 13, 14, 15, 16] . For this analysis, CDC Influenza Virus Surveillance was  used as the reference standard to which Google Flu Trends and  CDC ILI Surveillance were compared.  The study period was September 28, 2003 through May 17,  2008. These dates were chosen to include all available Google Flu  Trends historical ILI estimates and exclude the 2009 H1N1  pandemic which began during the 2008-09 influenza season. Analyses were performed by ''influenza season,'' defined as the period from July 1 through June 30 of the subsequent calendar year. As done in similar analyses [2] , we restricted our analysis to the period during which CDC influenza surveillance is intensified, from calendar week 40 through calendar week 20 of the subsequent year.