Because of the close anatomic relationship of the pericardium, diaphragm, and peritoneal cavity, a benign pneumoperitoneum may occur following median sternotomy incision and cardiac surgery. The pneumoperitoneum results from inadvertent opening of the peritoneal cavity during the initial surgical incision or during the subsequent cardiac surgery. It is generally of no significance to the patient, although it must be distinguished from pneumoperitoneum occuring secondary to intraabdominal pathology or to retrograde dissection of extraalveolar air in patients on ventilatory support. Two illustrative cases are presented.