In South Korea, the first patient with 2009 H1N1 virus infection was reported in May, 2009. The number of patients gradually increased until mid-October, when the number of patients was overwhelming for a month, and then gradually decreased, with few people becoming infected after February, 2010. Owing to sensational reports of childhood fatality in the mass media and a new diagnostic tool, the real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we had the opportunity to evaluate patients with 2009 H1N1 virus infection from the onset of their illness. In addition, during the study period we observed a dramatic effect of early treatment with corticosteroids and oseltamivir for patients with severe pneumonia including rapidly progressive pneumonia [9, 10] . In this study, we evaluated the epidemiological, clinical and laboratory features of children with 2009 H1N1 virus infections in a single hospital throughout the epidemic.