Inclusion Criteria:
* Be female, aged 18 to 49 years, inclusive.
* Have moderate to severe pelvic pain due to endometriosis.
* Have a history of regular menstrual cycles.
* Have been surgically (laparoscopy or laparotomy) diagnosed with endometriosis within 8 years of the start of screening.
* Have a Body Mass Index (BMI) of 18 to 36 kg/m², inclusive.
* Agree to use two forms of non-hormonal contraception during the study.
Exclusion Criteria:
* Are currently receiving gonadotropin-releasing hormone (GnRH) agonist, a GnRH antagonist other than NBI-56418, or danazol or have received any of these agents within 6 months of the start of screening.
* Are currently receiving subcutaneous medroxyprogesterone acetate (DMPA-SC) or intramuscular medroxyprogesterone acetate (DMPA-IM) or have received any of these agents within 3 months of the start of screening.
* Are currently using hormonal contraception or other forms of hormonal therapy or received such treatment within the last month.
* Have had surgery for endometriosis within the last month.
* Have had a hysterectomy or bilateral oophorectomy.
* Are using systemic steroids on a chronic or regular basis within 3 months.
* Have uterine fibroids ≥ 3 cm in diameter.
* Have pelvic pain that is not caused by endometriosis.
* Have unstable medical condition or chronic disease.
* Have been pregnant within the last six months.
* Currently breast feeding.
