In this study, we found that children of all ages except infants had a relatively even rate of infection with the 2009 H1N1 influenza virus. This is in agreement with other studies showing that few children and young adults have immunity against a new viral infection [2, 11, 12] . Our study of all 2009 H1N1 virus-infected children, in a single hospital, throughout the epidemic may have epidemiological implications, along with other studies based on data from admitted patients or gathered during a portion of the pandemic period [11] [12] [13] [14] . It has been reported that younger children (< 4-5 years old) with H1N1 virus infection tended to be admitted to hospital more frequently than older children or adults, and our results are compatible with these studies [2, 3, 6] . It has also been reported that certain risk factors are related to the likelihood of developing severe illness, including a younger age and particular underlying diseases, including obesity [6, 7, 15, 16] . In this study, we did not analyze obesity and young age as risk factors, and only 17 patients (7.8%) had underlying diseases. Although the hospitalization rate was higher among the younger children (0-4 years of age), in this study, pneumonia and severe pneumonia were more prevalent in the 5-9-year-old group than in younger children.