Catamenial pneumothorax is an extremely rare condition that affects women. Pneumothorax is the medical term for a collapsed lung, a condition in which air or gas is trapped in the space surrounding the lungs causing the lungs to collapse. Women with catamenial pneumothorax have recurrent episodes of pneumothorax that occur within 72 hours before or after the start of menstruation. The exact cause of catamenial pneumothorax is unknown and several theories have been proposed. Some cases are associated with the abnormal develop of endometrial tissue outside of the uterus (endometriosis), although the exact nature of this relationship in these cases is unknown.