OBJECTIVES To conduct a systematic review of randomised controlled trials comparing effectiveness of GnRHa in the treatment of endometriosis in order to assess the feasibility of conducting a meta-analysis and indirect treatment comparison. METHODS Inclusion criteria were all published randomized controlled trials comparing GnRHa versus any other pharmacological treatment or placebo in the treatment of laparoscopically confirmed moderate to severe endometriosis. Electronic databases were searched up to February 2014 (Embase, Medline, Cochrane, Clintrials.gov and clinical conferences). Search results were screened, eligible studies were assessed for risk of bias and data extracted. Studies were evaluated for the feasibility of inclusion into a network meta-analysis. RESULTS A total of 1,670 titles and abstracts were screened.  Sixty-nine papers relating to 62 studies were identified as eligible for inclusion. Fourteen of the 62 were excluded due to high risk of bias, a further 21 did not present sufficient data. The remaining 27 studies showed clinical heterogeneity in the measurement of disease severity and efficacy outcomes and quality of reporting.  Efficacy outcomes were measured using various methods and at different time points (3, 6 to 12 months). All but one measured pain, 12 using modified versions of the Biberoglu and Behrman (B&B), 7 using a similar method and 4 used a visual analogue scale. Quality of life was measured in 7 studies, most commonly by a generic HRQoL instrument, the SF-36.  Adverse event recording varied. CONCLUSIONS A  core set of studies were identified that could be incorporated into a quantitative analysis assessing effectiveness of GnRHa for treatment of moderate to severe endometriosis over 6 months. A network meta-analysis would potentially be feasible using more commonly reported pain outcomes, (modified) B&B; HRQoL and reported adverse events.