Abstract Introduction Endometriosis is when endometrial glands and stroma develop outside of the uterus and the most common extra-pelvic site is the involvement of the thoracic cavity. Thoracic endometriosis most commonly presents with catamenial pneumothorax, which refers to a temporal association with menstruation. The pathophysiology is unknown, but thoracic endometriosis should be suspected in any female of reproductive age presenting with catamenial pneumothorax. Case Description A 35 year-old female with a history of a total abdominal hysterectomy for uterine leiomyoma presented with chest pain, scapular pain, and shortness of breath. Her past medical history was notable for recurrent bilateral spontaneous pneumothoraces with prior pleurodeses. Chest x-ray on admission confirmed bilateral pneumothoraces. She was hemodynamically stable and saturating well on room air. There was no tension physiology. She underwent bilateral chest tube placement with successful lung re-expansion followed by bilateral pleurodesis. Pelvic MRI revealed endometriosis involving the vaginal cuff, ovaries, and tethering of the rectosigmoid colon with findings suspicious for bowel invasive disease. Previous imaging had failed to provide evidence for thoracic endometriosis, nor was endometriosis identified on previous VATS procedures. There was no evidence of cystic lung disease on prior imaging or other cause to explain recurrent pneumothoraces. She was started on hormonal suppressive therapy per gynecology for thoracic endometriosis syndrome. Discussion The diagnosis of thoracic endometriosis is usually made clinically; histopathologic confirmation is ideal but not required. The clinical characteristics that set thoracic endometriosis apart from other diseases that present similarly include temporal association with menstruation, right lung predominance, and demographics (i.e. young, female with recurrent disease). Patients with thoracic endometriosis may have higher serum carbohydrate antigen (CA) 125 and CA 19-9, however, these markers are poorly sensitive and non-specific. At times, pleural fluid is sent for evaluation of endometrial cells, but the diagnostic yield is thought to be low. Hormone suppressive therapy should be used to treat the majority of patients with pneumothoraces related to endometriosis, however, even with the combined strategy of hormonal suppression and surgery, recurrence can still occur.