The authors report the case of a two-year-old patient, hospitalized urgently with respiratory distress, radiologically establishing the diagnosis of subtotal pneumonia on the right complicated by the exudative-fibrinous pleurisy. CT showed the presence of a massive closed fluid formation, located subpleurally in the right hemi thorax (closed pleurisy), and associated with subtotal passive atelectasis of the right lung with no signs of destruction of lung tissue. After the installation of the intrathoracic drain, the fluid collection persisted. After thoracoscopy, surgery was performed, the patient undergoing a latero-posterior thoracotomy on the right with radical resection of the lymphangioma, which came from the anterior mediastinum. Histopathological and immunohistochemical examination confirmed the diagnosis. Conclusion. Mediastinal cystic lymphangioma in children can mimic a pleural effusion, which causes certain errors in surgical conduct. The final diagnosis is confirmed only by histological examination