A 12-year-old victim of an automobile-pedestrian accident appeared to develop severely compromised cardiac output shortly after intubation and positive pressure ventilation. Anteroposterior and lateral chest films showed air within the pericardial sac. After pericardiocentesis and withdrawal of air, cardiac function improved markedly as evidenced by a rise in blood pressure and a slowing of the pulse. A catheter was left in the pericardial sac for several days. The patient remained hemodynamically stable throughout the hospital stay and was subsequently discharged. Documentation of this degree of tamponade from air in the pericardium is quite uncommon.