© 2021. Thieme. All rights reserved.Objective: Many studies showed that less-severe disease symptoms and fewer mortality rates have been reported in pediatric novel coronavirus disease 2019 (COVID-19) patients. In this study, we aimed to reveal the relationship between the volume of thymus gland, which provides T lymphocyte maturation in children, with the severity of lung involvement and blood laboratory values in pediatric patients with COVID-19 infection. Methods: Thymus density and thymus and cardiac volumes were measured in pediatric COVID-19 patients and a control group that underwent thoracic tomography for reasons other than infection. Thymus/heart ratios were calculated to index the thymus volumes of the patients to their body dimensions. The severity of pneumonia was demonstrated by proportioning the involved lung parenchymal volume to the total lung volume in patients with typical involvement in thoracic tomography. The relationship between volumetric and blood laboratory values was statistically evaluated. Results: Thymus density (p = 0.015) and thymus/heart ratio (p = 0.04) significantly differed between patients with COVID-19 infection and the control group. A correlation was observed between the pneumonia involvement rate and C-reactive protein (CRP) (k: 0.451, p = 0.08) and white blood cell (WBC; k: 0.419, p = 0.015) values in the thoracic tomography of the COVID-19 group. Conclusion: The thymus gland is enlarged as an indicator of activation in COVID-19 infection. We hope that our study will guide new studies on the prognostic value of thymus size in lymphopenic patients with severe disease.