Endometriosis is another etiology for spontaneous hemothorax or hemopneumothorax. This is seen in menstruating women and may present as symptoms, signs, and radiographic changes that recur in the chest concurrent with menses. Such phenomena, termed catamenial by Dr. Lilington in 1972, may take on a number of clinical manifestations, including pneumothorax, hemothorax, hemoptysis, pulmonary nodules, chest pain, dyspnea, and others.93 Two thirds of these patients have a pathologic diagnosis of thoracic endometriosis on surgery, possibly due to cyclical changes in pathology with menstruation. Catamenial hemothorax represents a common manifestation of thoracic endometriosis syndrome, occurring in 14% of known cases, and concomitant pelvic endometriosis was found in 100% of cases.94 It is unilateral and affects the right side > 80% of the time; however, bilateral hemothorax from thoracic endometriosis has been reported.95 The majority of these patients are young, nulliparous, black women.96 Patients may be hemodynamically stable with an effusion. The etiology of thoracic endometriosis is not clear but includes possible transdiaphragmatic migration of endometrial tissue via naturally occurring fenestrations and occurring more frequently on the right side. Hematogenous or lymphatic spread after uterine manipulation from pelvic procedures is another possibility. Diagnosis is often delayed for months or even years, and a high index of suspicion is necessary.