Mannose-binding lectin (MBL) has broad range of activity against viruses through the mechanisms of neutralization, opsonization, and complement activation. Prior studies have demonstrated that MBL inactivated the season's influenza virus. Due to the fact that children have no neutralizing antibody against H1N1 2009 virus, innate immunity may be crucial in the defense against influenza. Therefore, we studied whether MBL levels played a role in H1N1 2009 infection in children. In a prospective survey, we revealed that MBL levels in ICU influenza cases were significantly lower than in children with influenza from infection disease ward. MBL may be involved in innate immune responses to H1N1 2009 infection in children.