Gonadotropin-releasing hormone (GnRH) agonists are increasingly used in ovarian hyperstimulation protocols in in vitro fertilization (IVF) programs. From March 1992 to June 1993, 565 patients attending our Institute underwent superovulation in 1104 IVF program cycles. Of these cycles, 650 were stimulated with clomiphene citrate and gonadotropins (human menopausal gonadotropin/hMG), and 454 with the GnRH agonist buserelin and hMG in a group of patients who had earlier failed to respond or did not conceive after clomiphene citrate/hMG stimulation. The ovarian response was similar in both groups, however, with the use of buserelin more oocytes were recovered -4.9 +/- 3.2 and 3.5 +/- 2.3 oocytes, respectively. The clinical pregnancy rate per transfer in the group of patients superovulated with buserelin/hMG was twice that of the clomiphene citrate/hMG group (21.0% vs. 10.4%). The relatively high pregnancy rate with the buserelin/hMG regimen in the group of 'poor responders' may be connected with GnRH agonist-induced pharmacological hypophysectomy and the sequelae thereof: normalization of some endocrinopathies, absence of an endogenous luteinizing hormone (LH) surge and better endometrium receptivity, oocytes and embryo quality.