A new closed-system device for the diagnosis and treatment of pneumothorax was evaluated in ten New Zealand white rabbits and compared with and open-system needle. The closed-system device proved to be safe for diagnostic thoracentesis. There were no pneumothoraces as a result of the procedure with the closed-system device as confirmed by chest roentgenograms and pleural pressure measurements. In contrast, 70% of the diagnostic thoracenteses with the open-system needle were associated with pneumothorax documented by x-ray films and a significant increase in mean pleural pressure. The new apparatus was more efficacious for evacuation of pneumothoraces because complete air removal occurred in 90% of the rabbits as compared with 60% of trials with the open-system needle. If the efficacy of the new closed-system device proves to be good in human infants, the pediatrician encountering a tension pneumothorax in the newborn can use a completely assembled system that is safe for diagnosing and treating this acute life-threatening condition.