The opening of schools in the late summer of 2009 may have triggered the fall wave of pandemic influenza A(H1N1) in the United States. We found that an elevated percentage of outpatient visits for influenza-like illness occurred an average of 14 days after schools opened in the fall of 2009. The timing of these events was highly correlated (Spearman correlation coefficient, 0.62; P<.001). This result provides evidence that transmission in schools catalyzes community-wide transmission. School opening dates can be useful for future pandemic planning, and influenza mitigation strategies should be targeted at school populations before the influenza season.