A 65 year old female patient developed a large left pleural effusion and a sternal split dehiscence following aorto-coronary artery bypass grafting. A second operation was performed to investigate and drain the pleural effusion and to repair the sternum. Subsequent to this operation the patient was in acute respiratory failure due to bilateral phrenic palsy. It is probable that the left phrenic nerve was damaged in the initial operation and the right nerve in the second. The patient's subsequent progress is described.