Congenital defects of the pericardium are rare. This report describes a young woman with a congenital complete pericardial defect who developed a giant pulmonary cyst. After operation the patient experienced chest pain caused by myocardial ischemia due to cardiac displacement. It is important to note that heart lability in patients with congenital pericardial defects may cause grave complications after thoracotomy associated with volume loss of the residual lung.