Nineteen women were treated with the gonadotropin-releasing hormone (GnRH) agonist buserelin in order to suppress the pituitary prior to gonadotropin treatment. Eight women were oligomenorrheic, 6 had polycystic ovarian disease (PCOD) and 5 women had normal cycles. Buserelin was administered for 3 weeks before ovarian stimulation, and the pituitary down-regulation was proven by provocative tests. Ovarian stimulation was then achieved by human menopausal gonadotropin (hMG) 2 ampules a day. Several abnormal responses to the combined buserelin/hMG treatment were noted in some patients. This included a sudden decrease in E2 level without LH surge (2 patients), induced follicular growth with buserelin instead of ovarian suppression (2 patients) and ovarian hyper-stimulation syndrome in 3 patients with PCOD. From this we conclude that although pituitary suppression can easily be achieved by GnRH analog administration, this does not ensure the prevention of unwanted responses. It is possible that the common denominator for these abnormal responses is that they are ovarian in origin, hence they occur in spite of pituitary down-regulation. Close monitoring of the suppression and stimulation stages will detect most cases of such failures. Furthermore it is possible that not all patients are suitable for the combined treatment of gonadotropin and GnRH agonist.