These data were from two separate A22 hydrochloride surveillance networks: Outpatient Influenza-like Illness Surveillance Network and the US Influenza Virologic Surveillance System. 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. 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. 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. 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�C09 influenza season. Analyses were performed by ����influenza season,���� defined as the period from July 1 through June 30 of the DIM-C-pPhtBu subsequent calendar year. As done in similar analyses, 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. For the primary analysis, scatter plots with least square regression lines were constructed to compare Google Flu Trends and CDC ILI Surveillance to the standard reference surveillance. Pearson��s correlation coefficients with 95% confidence intervals were then computed from these comparisons. Subsequently, additional correlation coefficients with 95% CI were calculated from surveillance comparisons by influenza season, US Census Region, and influenza season categorized by US Census Region. These subset analyses were summarized with mean correlation coefficients and standard deviations. Next, because Flu Trends was previously found to lead CDC ILI Surveillance observations by one to two weeks, we undertook additional correlation analyses to determine whether Google Flu Trends or CDC ILI Surveillance had a stronger correlation with CDC Virus surveillance data for the subsequent one or two weeks.