Objective: The aim of this study was to estimate the efficacy of elagolix, an oral gonadotropin-releasing hormone (GnRH) antagonist, for the treatment of endometriosis-associated pelvic pain. Methods: This was a phase II, randomized, placebo-controlled parallel group study conducted at 37 US centers, consisting of an 8-week double-blind period followed by a 16-week open-label period. Patients were 137 women aged 18 to 49, with laparoscopically confirmed endometriosis and moderate to severe nonmenstrual pelvic pain and dysmenorrhea, who were administered elagolix 150 mg daily or placebo. The primary outcomes of the study were the daily assessment of dysmenorrhea, nonmenstrual pelvic pain and dyspareunia using a modified Biberoglu-Behrman scale. Results: During the double-blind period, there were significantly greater mean reductions from baseline to week 8 in dysmenorrhea (-1.13 ± 0.11 vs. -0.37 ± 0.11, p<0.0001), nonmenstrual pelvic pain (-0.47 ± 0.07 vs. -0.19 ± 0.07, p = 0.0066), and dyspareunia scores (-0.61 ± 0.10 vs. -0.23 ± 0.10, p = 0.0070) in the elagolix group compared with placebo. Continued improvements were observed during the open-label treatment regardless of initial treatment allocation. Elagolix treatment was also associated with significant improvements in quality-of-life measures during the double-blind and open-label periods. The most common adverse events occurring with elagolix were nausea, headache and hot flush, each in 9.9% of patients. Conclusion: Elagolix effectively reduced endometriosis-associated pelvic pain over a 24-week period and was well-tolerated. © 2013 Wichtig Editore.