We report a patient with complete left pericardial defect whose phrenic nerve, split into two portions, passes both ventral and dorsal to the defect. The dorsal part of the phrenic nerve passes over the ventral surface of the pulmonary artery and veins, indicating that the pericardio-pleural foramen has been obliterated. Contrary to the widely accepted embryogenic theory that pericardial defect results from persistence of the pericardiopleural foramen, we consider that the defect in this patient resulted from a tear in the pericardio-pleural membrane immediately lateral to the common cardinal vein.