The clinical severity of the 2009 pandemic H1N1 influenza (H1N1 pdm09) was thought to be related to the difference between the amount of viral load and condition of the host immune response. We investigated the role of serum levels of IgG and its subclasses in clinical severity using the data from 45 child inpatients suffering from bronchitis or mild pneumonia caused by possible H1N1 pdm09 (pH1N1 pdm09) infection. After selecting parameters for serum IgG subclasses and logarithmically transformed urinary beta-2 microglobulin/creatinine (b2MG/Cr) values and admission duration, we performed path analysis using a mean covariance structure equation analysis to investigate the relationship between the clinical severity and the foregoing selected parameters. Total path analyses using a Bayesian method revealed that the estimated clinical severity caused by pH1N1 pdm09 was positively associated with maximal respiration rates, admission duration, and log urinary b2MG/Cr levels, whereas negatively associated with serum IgG, IgG1, IgG2, and IgG3 levels, duration of neuraminidase inhibitor therapy in outpatient clinics, and age. Serum IgG and its subclasses (IgG1-IgG3) reduced estimated clinical severity in children with pH1N1 pdm09 infection.