Gonadotrophin-releasing hormone (GnRH) antagonists have recently been introduced into clinical practice. They appear to offer a promising alternative to the long-established GnRH agonist regimens for prevention of a premature LH surge during ovarian stimulation for assisted reproductive techniques. Clinical outcomes achieved with antagonists are comparable with those of a long GnRH agonist protocol, while treatment times and gonadotrophin requirements are reduced and safety is improved. In particular, the antagonists appear to be associated with a lower risk of ovarian hyperstimulation syndrome (OHSS) than do agonists. Patient surveys suggest a preference for antagonist over agonist treatment cycles. These benefits suggest that GnRH antagonists have the potential to replace agonists as the treatment of choice in ovarian stimulation for assisted reproductive techniques. Two agents, cetrorelix and ganirelix, are currently in clinical use. Cetrorelix is available in single- and multiple-dose formulations, offering increased flexibility compared with ganirelix.