A total of 739 (225 H1N1(+)) children with a diagnosis of acute respiratory infection were hospitalized during July to December 2009. The H1N1(+) children were compared with 225 randomly enrolled H1N1(-) children with an influenza-like illness. As compared with influenza-like illness patients, patients with 2009 influenza A/H1N1 were characterized by older age, more vomiting, less hypoxemia and wheezing, lower white blood cell counts, less neutrophilia, and severe lymphopenia.