This study investigated the relationship between school session status and H1N1 influenza prevalence. Weekly means of Influenza-like-Illness (ILI) rates over the period May 1 to October 31, 2009 were compared between areas where schools were and were not in session in the United States. Rates were substantially and significantly higher in areas where schools were in session. This result held separately in spring and fall and was robust to various controls.