Résumé : Background: The objective of this randomized controlled trial was to assess the effect of oral contraceptive pill (OCP) pretreatment on the probability of ongoing pregnancy in patients treated with a GnRH antagonist for IVF. Methods: A fixed dose of 200 IU recombinant FSH (rF SH) was started in 425 patients either on day 2 of the menstrual cycle (non-OCP group: n = 211) or 5 days after discontinuing the OCP (OCP group: n = 214). GnRH-antagonist was initiated on day 6 of stimulation, and triggering of final oocyte maturation was performed with 10,000 IU of HCG. Results: Ongoing pregnancy rates per started cycle in the non-OCP and OCP group were 27.5% and 22.9%, respectively [95% confidence interval (CI) of the difference: -3.7 to +12.8]. Pregnancy loss was significantly increased in the OCP (36.4%) compared with the non-OCP group (21.6%) (95% CI of the difference: -28.4 to -2.3). Conclusion: Pretreatment with OCP, as compared with initiation of stimulation on day 2 of the cycle in patients treated with GnRH antagonist and recombinant FSH, appears to be associated with a not significant difference in ongoing pregnancy rates per started cycle and results in a significantly higher early pregnancy loss. © The Author 2005. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved.