We retrospectively evaluated all patients with 2009 H1N1 virus infection during the pandemic (2,971 patients) for epidemiologic characteristics, and for clinical characteristics, we reviewed the medical records and chest radiographic findings of 217 children admitted to The Catholic University of Korea, Daejeon St Mary's Hospital between 1 September 2009 and 31 January 2010. The diagnosis of patients depended on positive results for the 2009 H1N1 virus RT-PCR (AccuPower ™ in Korea, BiONEER, Alameda, CA, USA) through throat swabs.