Five patients with catamenial pneumothorax are reported, four of whom had proven diaphragmatic defects. Two had microscopic diaphragmatic endometrial implants in the area of the defects. No specimen was obtained in two others. A fifth patient with the condition, as well as with extensive pelvic endometriosis, was treated successfully by closed thoracostomy followed by hysterectomy and salpingo-oophorectomy. Patients with recurrent catamenial pneumothorax should be assumed to have diaphragmatic defects and be considered for treatment by thoracotomy and closure of these defects if the severity of the condition warrants it.