Pneumothorax (pl. pneumothoraces) is an abnormal collection of air or gas in the pleural space that separates the lung from the chest wall, and that may interfere with normal breathing.

A primary pneumothorax is one that occurs without an apparent cause and in the absence of significant lung disease, while a secondary pneumothorax occurs in the presence of pre-existing lung pathology. Occasionally, the amount of air in the chest increases markedly when a one-way valve is formed by an area of damaged tissue, leading to a tension pneumothorax. This condition is a medical emergency that can cause steadily worsening oxygen shortage and low blood pressure. Unless reversed by effective treatment, these sequelae can progress and cause death.

Pneumothoraces can be caused by physical trauma to the chest (including blast injury), or as a complication of medical or surgical intervention. Symptoms typically include chest pain and shortness of breath. Diagnosis of a pneumothorax by physical examination alone can be difficult or inconclusive (particularly in smaller pneumothoraces), so a chest X-ray or computed tomography (CT) scan is usually used to confirm its presence.