Primary idiopathic chylopericardium is a rare disease, particularly in children. We report the case of a 5-year-old male child who presented with cough and dyspnoea. Bulging of the chest wall and distant heart sounds suggested the diagnosis of a pericardial effusion, which was confirmed on a chest computed tomography. Pericardial drainage confirmed the diagnosis of a chylopericardium. After the failure of continuous catheter drainage, surgical ligation of the thoracic duct and the creation of a pericardial window were performed under thoracoscopy.