Four cases of congenital defect of the pericardium are described; three had complete absence of the left pericardium and one had a partial defect. One of the patients developed acute pulmonary oedema in the immediate post-operative period following repair of a sinus venosus defect. This complication was thought to be directly related to the pericardial defect. The functions of the pericardium are discussed. The clinical features of the different types of pericardial defect are described. The diagnosis is usually made from the radiographic appearances. Electrical alternans has been noticed in one patient. This is probably due to the increased mobility of the heart. These defects are not completely benign, as has been suggested before. They can cause disabling symptoms, as in case 3, and may be directly responsible for death.