Research questionDoes 3-months of gonadotrophin releasing hormone agonist (GnRHa) treatment before IVF improve clinical pregnancy rate in infertile patients with endometriosis? DesignSingle-blind, placebo-controlled clinical trial of 200 infertile women with endometriosis assigned to use GnRHa (study group) or placebo (control group) for 3 months before IVF. Clinical, embryological outcomes and stimulation parameters were analysed. Clinical pregnancy rate was the primary endpoint. In a subgroup of 40 patients, follicular fluid levels of oestradiol, testosterone and androstendione were measured. Gene expression profile of CYP19A1 was analysed in cumulus and mural granulosa cells. ResultsImplantation or clinical pregnancy rate were not significantly different between the two groups. Clinical pregnancy rates were 25.3% and 33.7% in the study and control groups, respectively (P = 0.212). Cumulative live birth rate was not significantly different: 22.0% (95% CI 13.0 to 31.0) in the study group and 33.7% (95% CI 24.0 to 44.0) in the control group (P = 0.077). Ovarian stimulation was significantly longer and total dose of gonadotrophins significantly higher in the study group (both P < 0.001). Serum oestradiol levels on the day of HCG were significantly lower in the study group (P = 0.001). Cancellation rate was significantly higher in the study group (P = 0.042), whereas cleavage embryos were significantly more numerous in the control group (P = 0.023). No significant differences in the expression of CYP19A1 gene in mural or cumulus granulosa cells or steroid levels in follicular fluid between the two groups were observed, but testosterone was significantly lower in the study group (P < 0.001). ConclusionThree-months of GnRHa treatment before IVF does not improve clinical pregnancy rate in women with endometriosis.