Inclusion Criteria:  Female with surgically confirmed endometriosis Clinical decision to begin use of the LNG-IUD Age 13 to 24 years Current pelvic pain (score ≥ 3 on Visual Analog Scale) that has been present for at least 2 months prior to enrollment Willingness to comply with visit schedule and study protocol  Exclusion Criteria:  Pre-menarche or post-menopause Previous gonadotropin-releasing agonist use Contraindications to an IUD (e.g., cavity-distorting uterine anomaly, acute pelvic inflammatory disease, <3 months from postpartum endometritis or septic abortion, local neoplasm, uterine bleeding of unknown etiology, breast cancer, untreated cervicitis or vaginitis or other lower genital tract infections) Active or historical venous thromboembolism, active or recent arterial thromboembolic disease Impaired liver function or liver disease Systemic lupus erythematosus Uncontrolled diabetes or uncontrolled hypertension (BPs >140s/90s) Hypersensitivity to any component of the NETA or LNG-IUS Concurrent use of moderate or strong CYP34a inducers or inhibitors (such as erythromycin, ketoconazole, phenobarbital, rifampin) Significant mental or chronic systemic illnesses that may impact pain assessment Breast feeding, giving birth within the last 6 months, pregnancy, or planning to become pregnant in the next 12 month