A 15-year-old pregnant girl developed severe substernal pain during the final effort to give birth to her first child. Shortly thereafter a growing crepitant suprasternal swelling was noted. Pneumomediastinum and subcutaneous emphysema was clearly demonstrated by a chest film showing mediastinal air dissecting towards the neck and supradiaphragmatically. The causes and physiopathology of spontaneous pneumomediastinum are reviewed. The therapeutic use of oxygen is discussed.