Gonadotropin-releasing hormone agonists are commonly used to achieve pituitary and ovarian downregulation in preparation for in vitro fertilisation. The purpose of this study was to examine the efficacy of gonadotropin-releasing hormone agonists in achieving downregulation prior to the commencement of ovarian stimulation. After a three week pretreatment with the gonadotropin-releasing hormone agonists, a satisfactory response as determined by ultrasound, was achieved in 164/200 cycles (82.0%). Treatment was continued for up to five additional weeks in 35 of the remaining 36 cases. Ovarian suppression was achieved in nine of 11 cases after an additional week of treatment, in nine of 14 cases after two weeks and in two of 10 in the remainder. The results of this study suggest that downregulation will be achieved in over 80% of cases after a three week pretreatment with a gonadotropin-releasing hormone agonist, and a short prolongation of treatment may be worthwhile in those cases where downregulation is not achieved in the first instance.