CDC identified the first case of 2009 H1N1 pandemic influenza on April 15, 2009. During the first 3 months of the outbreak, approximately 43,000 cases were reported to CDC. In June 2009, the World Health Organization declared the outbreak an influenza pandemic. Because no existing influenza surveillance system in the United States monitored influenza-like illness (ILI) among persons with ILI who did not seek health care, CDC initiated community-based surveillance of self-reported ILI (defined as the presence of fever with cough or sore throat) and health-care--seeking behavior through a supplementary module of the Behavioral Risk Factor Surveillance System (BRFSS). This report summarizes results from BRFSS surveys conducted during September 2009--March 2010. Among 216,431 adults and 43,511 children (aged <18 years), the average monthly percentage of respondents reporting ILI in the 30 days preceding the interview was 8.1% among adults (range: 5.5% for September interviews to 9.5% for November) and 28.4% among children (range: 20.4% for September interviews to 35.9% for November). Health care was sought by 40% of adults and 56% of children with self-reported ILI. The results indicate that reported symptoms of ILI were widespread during the 2009--10 influenza season, with a substantial percentage of those reporting ILI seeking health care.