Six patients referred for echocardiographic evaluation in whom an unusual echocardiographic sign resulted from air within the mediastinum or pericardium are described. Three patients had a pneumomediastinum that occurred after chest trauma and three patients had a pneumopericardium induced during a therapeutic pericardiocentesis. Important features included a broad band of echoes (air) recorded during held respiration which obscured the normal cardiac structures and dropout (gap) of echoes posteriorly. Between the cyclic appearance of the "air gap" sign, intracardiac structures were normally visualized. Echocardiographic recording of the air gap sign was identical in the six cases; it disappeared after resolution of clinical signs and symptoms of the pneumopericardium or pneumomediastinum. The pattern most likely resulted from air within the anterior mediastinum or pericardium interfering with the echographic beam and resulted in a cyclic appearance from systole to early diastole as the air was displaced by the changing cardiac size. Recognition of the air gap sign can be helpful in evaluating patients for pneumomediastinum or pneumopericardium after thoracic trauma.