A 38 year old woman with congenital partial absence of the left pericardium is presented. The condition is fairly rare, usually diagnosed incidentally during intrathoracic operations or at autopsy. Eleven operatively corrected cases are reported in the literature. The present case was admitted because of transient attacks of chest pain, palpitation and dyspnoea. These attacks were brought on when she lay on her left side and were promptly relieved by a change of position. The chest radiograph revealed a prominence of the superior aspect of the left heart border and a slight laevo-position of the heart. A diagnostic pneumothorax on the left side confirmed the diagnosis of a pericardial defect. The defect was repaired by pericardioplasty. Operative correction of a partial left pericardial defect is indicated because of the danger of luxation of the heart out of the pericardium and sudden death. This has been reported, as well as a death caused by spreading of infection from the pleural cavity into the pericardium and heart.