Two unusual instances of pneumothorax complicated by subcutaneous emphysema have been observed. In the first patient, generalized subcutaneous emphysema and a left pneumothorax followed an attempt at an intracardiac injection of epinephrine. A review of the literature does not reveal a similar complication, although Graham1noted an infant who developed a left pneumothorax subsequent to an intracardiac puncture at birth. In the second patient, these phenomena were observed after the aspiration of a pleural effusion. It is generally held that a pneumothorax is produced by the aspiration of air from the ruptured lung into the intrapleural space during inspiration. While this explanation may account for the ordinary pneumothorax, in which the intrapleural pressure is negative or atmospheric during the inspiratory phase, it does not satisfactorily apply to a tension pneumothorax, in which the pressure is positive during both phases. A consideration of the complete mechanism involved in the latter