Objective: To evaluate cycle outcomes in patients with either poor or normal prognosis undergoing IVF treatment with a GnRH antagonist (Ganirelix acetate) for LH suppression. Design: Nonrandomized, noncontrolled, retrospective review. Patient(s): 204 patients, aged 23-41 years, undergoing IVF. Intervention(s): Patients completed 225 consecutive cycles of IVF with a GnRH antagonist (Antagon; Organon, Roseland, NJ) for LH surge prevention. Sixty cycles were conducted in patients with a known poor prognosis, whereas 165 were conducted in patients with a normal IVF prognosis. Main Outcome Measure(s): Pregnancy rate (PR), for the series as a whole and according to prognosis, and serum E-2 patterns. Result(s): The PR per initiated cycle for the series as a whole was 33.3%. The pregnancy rate was 42.1% per ET for the entire series, with a cycle cancellation rate of 21%. When evaluated by prognosis, the poor-prognosis patients had PRs of 8.3% per attempt and 15% per transfer, whereas the normal-prognosis patients had PRs of 40% per attempt and 45% per transfer. Pregnancy rate did not vary by E-2 pattern (drop, plateau, or rise). Oral contraceptive pretreatment was noted to be associated with high cancellation rates in the group of known poor responders, whereas for the group as a whole, cycle outcome was unaffected by the use of oral contraceptives. Conclusion(s): Use of GnRH antagonists in patients with an a priori poor IVF prognosis results in predictably poor outcomes. Patients without factors predicting poor outcome have acceptable PRs. The pattern E-2 rise immediately after initiation of GnRH antagonists does not predict cycle outcome. Oral contraceptives can be successfully used to schedule antagonist-based IVF cycles but might increase the risk of cycle cancellation in some patient populations. (c) 2005 by American Society for Reproductive Medicine.