Introduction: H1N1 flu (swine flu) first emerged in mid-April 2009, and the Centers for Disease Control and Prevention (CDC) was a lead U.S. federal agency responding to the disease, the first pandemic in more than 40 years. Aim: The purpose of this program evaluation is to assess CDC’s Web-based outreach for the first part of the H1N1 flu pandemic (April to July 2009). Evaluation focuses on the seven CDC Web sites used for H1N1 flu outreach: H1N1 Flu (English and Spanish), CDC.gov, CDC en Español, CDC Partners, CDC Seasonal Flu, and MMWR. Methods: The evaluation assessed Web use and users through data collected by the American Consumer Satisfaction Index (ACSI) and specific Web statistics gathered through Omniture SiteCatalyst. Discussion: People have been using the internet to collect health information since the early days of popular usage. During the early H1N1 flu emergency, CDC used seven different Web sites to reach out to its users, who are primarily public health professionals, healthcare providers and consumers. Most users cite one of four reasons for visiting CDC’s Web sites: health information for my work, health information for myself, health information for patient or client, and health information for someone else. Between April and July, a clear majority of users visited the Web site for H1N1 flu information (from a high of 58% in May to a low of 35% in July). CDC.gov has received ACSI satisfaction scores of 80 (considered a top performer) or higher since the first quarter of 2007; the overall site satisfaction score of 82 during the first quarter of 2009 was bested by the H1N1-specific satisfaction score of 84 from mid-April to the end of June. Traffic to the site increased enormously during the early H1N1 flu emergency, from approximately 45 million page views per month in February and March 2009 to more than 98.2 million page views in April 2009. Recommendations: Those working on CDC.gov H1N1 flu-related content should continue trying to anticipate and meet user needs by expanding Web content, partnering with external organizations to share key information through syndication and information placement, and enhancing translation capabilities.