Résumé : Background: The aim of this study was to compare ongoing pregnancy rates per started cycle between patients randomized at consultation to have embryo transfer either on day 3 or on day 5 of in-vitro culture. Methods: All patients <43 years of age for whom IVF was indicated were allowed to participate in the study (day 3 group, 234 patients; day 5 group, 226 patients). Ovarian stimulation was performed either using GnRH antagonists/recombinant FSH (rFSH) (day 3, 70.1% of patients; day 5, 72.6% of patients) or using the long GnRH agonist protocol/urinary gonadotropins (day 3, 29.9% of patients; day 5 27.4% of patients). Results: The random decision to initiate a cycle leading to day 5 as compared with a day 3 transfer was associated with a significantly lower chance of embryo cryopreservation (day 3, 61.5%; day 5, 50.4%; P < 0.02). Ongoing pregnancy rate per started cycle did not differ between the two groups compared [day 3, 32.1%, 95% confidence interval (CI) 26.4-38.2%; day 5, 33.2%, 95% CI 27.3-39.5%]. Conclusions: Advising patients at consultation to initiate an IVF cycle leading to a day 5 as compared with a day 3 transfer does not appear to increase the probability of ongoing pregnancy, and is associated with a significantly lower probability of obtaining cryopreserved embryos. © European Society of Human Reproduction and Embryology 2004; all rights reserved.