Résumé : Background: The significance of suppressed LH levels in GnRH antagonist cycles for IVF outcome is currently unknown. The purpose of this study was to evaluate prospectively the association between LH levels and ongoing pregnancy achievement after GnRH antagonist initiation in IVF cycles. Methods: Ovarian stimulation with a fixed dose of 200 IU recombinant FSH and daily GnRH antagonist (ganirelix) 0.25 mg from day 6 of stimulation was initiated in 116 women. Patients were not pretreated with an oral contraceptive. Induction of final oocyte maturation was performed with HCG 10 000 IU as soon as three follicles of ≥17 mm were present in ultrasound, and was followed by oocyte pick-up, conventional IVF or ICSI, and embryo transfer. The luteal phase was supplemented with vaginal progesterone. Results: A significant decrease of both ongoing pregnancy rate and implantation rate was present across groups of patients with increasing LH levels. The highest implantation rate and ongoing pregnancy rate was present in those patients with LH levels on day 8 of stimulation ≤0.5 IU/l. Conclusions: Profound suppression of LH on day 8 of stimulation is associated with a significantly higher chance of achieving an ongoing pregnancy. More studies are necessary to evaluate this phenomenon further. © European Society of Human Reproduction and Embryology 2004; all rights reserved.