During the 2009-10 influenza season, the second wave of influenza activity from 2009 pandemic influenza A (H1N1) occurred in the United States; few seasonal influenza viruses were detected. Influenza activity peaked in late-October and was associated with higher pediatric mortality and higher rates of hospitalizations in children and young adults than in previous seasons. The proportion of visits to health-care providers for influenza-like illness (ILI), as reported in the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet), was among the highest since ILI surveillance began in 1997 in its current form. This report summarizes influenza activity in the United States during the 2009-10 influenza season (August 30, 2009-June 12, 2010).