Background. The aim of this study was to evaluate the response to treatment in a group of patients undergoing IVF and randomised to receive GnRH-antagonist or the GnRH-agonist. The endpoints were the pattern of follicular growth, the maturity of the oocytes collected, the embryo quality and the pregnancy outcome.

Methods. A total of 136 patients undergoing IVF were included. Sixty-seven patients were allocated to the GnRH antagonist and 69 patients to the GnRH agonist. GnRH antagonist was administered when the leading follicle reached a diameter of 12–14 mm. GnRH agonist was administered in a long luteal protocol.

Results. The mean numbers of oocytes retrieved and mature oocytes were significantly higher in the agonist than in the antagonist group (p < 0.02 and p < 0.01, respectively). Embryo quality, implantation rate, clinical pregnancy rates, ongoing pregnancy rate and miscarriage rate were similar in both groups.

Conclusions. Better follicular growth and oocyte maturation are achieved with GnRH agonist treatment. However, both regimens seem to have similar efficacy in terms of implantation and pregnancy rates. Further studies clarifying the effect of the GnRH antagonist on ovarian function are needed, as well as a clear definition of the best period of the follicular phase for the GnRH antagonist administration.