Catamenial pneumothorax is a spontaneous pneumothorax that recurs during menstruation, within 72 hours before or after the onset of a cycle. It usually involves the right side of the chest and right lung, and is associated with thoracic endometriosis. A third to a half of patients have pelvic endometriosis as well. Despite this association, CP is still considered to be misunderstood as is endometriosis considered to be underdiagnosed. The lack of a clear cause means that diagnosis and treatment is difficult. The disease is believed to be largely undiagnosed or misdiagnosed, leaving the true frequency unknown in the general population.

Catamenial pneumothorax is defined as at least two episodes of recurrent pneumothorax corresponding with menstruation. It was first described in 1958 when a woman presented with 12 episodes of right-sided pneumothorax over 1 year, recurring monthly with menstruation. Thoracotomy revealed thoracic endometriosis. Many patients present with chest pain close to their menstrual periods. Surgical exploration can be used in an attempt to visualize the problem; mechanical pleurodesis or hormonal suppressive therapy can also be used. Sometimes, a second surgical look can show fenestrations in the diaphragm. Due to the fact that endometriosis has been attributed to retrograde menstruation, upwards of 90% of women may have an immune deficiency. This prevents clearance of endometrial cells from the peritoneum.

Endometriosis is defined as tissue similar to the endometrial tissue that has implanted outside of the uterus. Mechanisms include retrograde menstruation resulting in abdomino-pelvic spread, blood-borne or lymphatic spread and deposition, and metaplasia.

Thoracic endometriosis is the most common non-abdominal site of involvement and is also the primary risk factor for catamenial pneumothorax. Catamenial pneumothorax is the primary clinical presentation of thoracic endometriosis, and is defined as recurrent episodes of lung collapse within 72 hours before or after menstruation.