Mediastinal emphysema may occur due to migration of air from the lungs, from the esophagus or tracheobronchial tree and from the abdomen. Of especial interest is the mechanism starting with the rupture of the perivascular alveoli due to a rapid decrease in intrathoracic pressure from any cause, the development of pulmonary interstitial emphysema and migration of the air into the mediastinum. In one case the patient had severe interstitial emphysema of the left lung, mediastinal emphysema and subcutaneous emphysema without pneumothorax and rapid improvement followed tracheotomy. In another case the patient had interstitial emphysema of the left lung that did not progress to mediastinal emphysema and subcutaneous emphysema. Pneumothorax was not present. Recovery was more rapid than in the first patient.