Abstract A one-way, plastic encased valve has been described as a substitute for the water-seal trap to be used with intercostal catheter drainage of the pleural space. This has proved effective in the following clinical circumstances: (1) in conjunction with definitive, intercostal catheter drainage treatment of spontaneous pneumothorax; (2) in conjunction with intercostal catheter drainage for traumatic hemopneumothorax; and (3) as a substitute for water-seal drainage in postoperative thoracotomy patients. This device is not anticipated as a substitute for the water-seal trap under ordinary circumstances at the present time. The primary usefulness would appear to be the reduction of the hazard of evacuating chest casualties during an emergency. For the purpose of investigation, patients requiring intercostal catheter drainage of the pleural cavity of air or fluid were selected for trial. A particular peace-time use for this valve would appear to be the surgical treatment of spontaneous pneumothorax on a semiambulant basis.