OBJECTIVES: To describe the impact of add-back therapy on adherence and surgery rates among endometriosis patients starting leuprolide acetate (LA) therapy. Hormonal add-back therapy is used in conjunction with LA treatment of endometriosis patients to reduce potential side effects associated with LA gonadotropin releasing hormone-agonist effects. METHODS: Truven Health MarketScan Commercial Encounters database was used to identify women aged 18-49 with endometriosis (ICD-9-CM code 617.xx) who initiated LA (index date) in 2005-2011. Women with 12 months of continuous enrollment pre- and post-index and no evidence of endometriosis-related surgeries pre-index or up to 30 days post- index, no pre-index use of estrogen or non-contraceptive hormones and no diagnoses of uterine fibroids, malignant neoplasms, infertility, or pregnancy were included in the analysis. Proportion of patients adherent to LA, measured by Medication Possession Ratio (MPR)≥0.80, and time to endometriosis-related surgery in the post-index period were compared between patients with no add back therapy, patients who used norethindrone/norethindrone acetate (NETA) add-back and patients who used other add-back (estrogens, progestins, or combinations) using logistic and Cox Proportional Hazard regression models controlling for demographics, comorbidities and pain medication use. RESULTS: Final study sample included 3,114 women with mean age 36.9 years. The majority of women did not use add-back (n=1,965, 63.1%) while 22.9% used LA with NETA (n=713) and 14% used other add-back (n=436). NETA patients had higher likelihood of being adherent to LA than other add-back patients (OR= 1.4, 95% CI (1.1, 1.8)) or non-add-back patients (OR=2.2, 95% CI (1.8, 2.6)). NETA patients had lower surgery rate in the 12-months post-index compared to other add-back (HR=0.62, 95% CI (0.46, 0.83) or non-add-back patients (HR=0.72, 95% (CI 0.57, 0.91)). CONCLUSIONS: Use of add-back therapy, particularly norethindrone/NETA add-back, was associated with improved adherence to LA and reduced rates of endometriosis-related surgery, which has substantial economic and patient burden.