In this study, in-vitro fertilization (IVF) cycles were examined in patients with endometriosis who received gonadotrophin releasing hormone agonist (GnRHa) therapy in an effort to improve their response to stimulation. Twelve patients treated with an identical gonadotrophin stimulation protocol, with and without GnRHa, were evaluated using paired analysis. In the GnRHa cycles, the total number of oocytes retrieved and transferred per cycle was higher than in the control cycles. In addition, use of the agonist lowered the cancellation rate from 33 to 0%, while a trend towards better pregnancy results was observed. When analysed according to the stage of the disease, the patients with stage III or IV endometriosis had a more dramatic improvement with GnRHa. These data suggest that GnRHa therapy may be beneficial in some patients with endometriosis undergoing IVF.