Purpose: This study was conducted to investigate the immune responses of children with moderate and severe novel influenza A virus (H1N1) pneumonia, and to compare their clinical and immunological findings with those of control subjects. Methods: Thirty-two admitted patients with H1N1 pneumonia were enrolled in the study. The clinical profiles, humoral and cell-mediated immune responses of the 16 H1N1 pneumonia patients who were admitted to the pediatric intensive care unit (severe pneumonia group), 16 H1N1 pneumonia patients admitted to the pediatric general ward (moderate pneumonia group) and 13 control subjects (control group) were measured. Results: Total lymphocyte counts were significantly lower in patients with H1N1 pneumonia than in the control group (P=0.02). The number of CD4+ T lymphocytes was significantly lower in the severe pneumonia group ($411.5{pm}253.5/{mu}L$) than in the moderate pneumonia ($644.9{pm}291.1/{mu}L$, P=0.04) and control ($902.5{pm}461.2/{mu}L$, P=0.01) groups. However, the number of CD8+ T lymphocytes was significantly higher in the severe pneumonia group ($684.2{pm}420.8/{mu}L$) than in the moderate pneumonia ($319.7{pm}176.6/{mu}L$, P=0.02) and control ($407.2{pm}309.3/{mu}L$, P=0.03) groups. The CD4+/CD8+ T lymphocytes ratio was significantly lower in the severe pneumonia group ($0.86{pm}0.24$) than in the moderate pneumonia ($1.57{pm}0.41$, P=0.01) and control ($1.61{pm}0.49$, P=0.01) groups. The serum levels of immunoglobulin G, immunoglobulin M and immunoglobulin E were significantly higher in the severe pneumonia group than in the 2 other groups. Conclusion: The results of this study suggest that increased humoral immune responses and the differences in the CD4+ and CD8+ T lymphocyte profiles, and imbalance of their ratios may be related to the severity of H1N1 pneumonia in children.