Traumatic lung pseudocyst is a lung injury due to closed chest trauma. The authors report one case in a 20 month-old boy. This cavitary lesion is underestimated since associated lesions often mask the pneumatocele. Its frequency is higher in children and young adults. Diagnosis can be assessed early when chest film shows the characteristic cavitary lesion. Computed tomography may be helpful in the recognition of paramediastinal traumatic lung pseudocyst. Since the lung pseudocyst usually involves, no aggressive therapy is justified.