The diagnosis of catamenial pneumothorax should be suspected whenever a women in the third or fourth decade of life has recurrent right-sided chest pain temporally associated with menstruation. This entity was first described in 1958 and since then, 59 cases have been reported in the English literature. There is no universally accepted explanation for the pathogenesis of the pneumothorax. Four additional cases are presented which support the significant role that diaphragmatic defects and endometriosis play in the pathogenesis of catamenial pneumothorax. At thoracotomy, numerous small perforations of the right diaphragm were found in three patients, two of whom had associated endometriosis of that diaphragm. The fourth patient had indirect evidence of diaphragmatic defects proven by a diagnostic pneumoperitoneum. On the basis of the hypothesis of transfallopian passage of air, this patient was treated by tubal ligation only and has not had a recurrence of pneumothorax since. This form of therapy has not been reported previously for this disorder.