OBJECTIVE: To evaluate the efficacy of the GnRH agonist (GnRH-a) nafarelin compared with placebo administered for 6 months after reductive laparoscopic surgery for symptomatic endometriosis. DESIGN: Randomized, prospective, placebo-controlled, multicenter clinical trial. SETTING: Thirteen clinics including private practice and university centers. PATIENT(S): One hundred nine women aged 18-47 with laparoscopically proven endometriosis and pelvic pain who had undergone reductive laparoscopic surgery for endometriosis. INTERVENTION(S): Patients were randomized to receive either the GnRH-a nafarelin (200 micrograms twice daily) or placebo for 6 months. MAIN OUTCOME MEASURE(S): Time to initiation of alternative treatment (the length of time from beginning study medication to receiving alternative therapy or to deeming that the study drug was ineffective) and patient-reported and physician-assessed pelvic pain scores. RESULT(S): The median time to initiation of alternative treatment was > 24 months in the nafarelin group versus 11.7 months in the placebo group. Fifteen (31%) of 49 nafarelin-treated patients required alternative therapy, compared with 25 (57%) of 44 placebo-treated patients. The patients' pelvic pain scores dropped significantly in the nafarelin and placebo groups after 6 months of treatment. Physician summary ratings showed significant improvement in the nafarelin group and no significant changes in the placebo group after 6 months of treatment. CONCLUSION(S): Compared with placebo, nafarelin administered after reductive laparoscopic surgery for endometriosis significantly delays the return of endometriosis symptoms requiring further treatment.