- Objective To compare blastocyst-stage embryo transfers (ETs) with day 2–3 ETs in patients who failed to conceive in three or more day 2–3 IVF/ET cycles. Design Prospective, randomized. Setting Fertility unit in a university medical center. Patient(s) Fifty-four patients with an adequate ovarian response underwent oocyte retrievals. The patients were prospectively and randomly divided into blastocyst-stage and day 2–3 ET groups. Intervention(s) Ovarian down-regulation was obtained using GnRH agonist, and controlled ovarian hyperstimulation was achieved using daily administration of gonadotropins. Main outcome measure(s) The rate of blastocyst formation, ET cancellations, pregnancies, implantation, multiple gestation, and live births. Result(s) The clinical pregnancy rates per oocyte retrieval were 21.7% and 12.9% per blastocyst and day 2–3 ETs, respectively. Although there was a significantly higher implantation rate for blastocyst embryos (21.2%) as compared with 48- to 72-hour embryos (6%), the multiple-pregnancy rate was not significantly different between both groups. An ET cancellation rate of 26% and 6.4% for blastocyst and day 2–3 ETs, respectively, was observed. The presence of two or more 8-cell embryos on day 3 in culture carried a high probability of obtaining blastocysts for transfer. Conclusion(s) This prospective randomized study suggests that in patients with an adequate ovarian response who failed to conceive in at least three IVF/ET cycles  transfer of blastocyst-stage embryos carries a significantly higher implantation rate;  the pregnancy rate per oocyte retrieval and ET are higher in the blastocyst-stage group, even if it did not reach statistical significance;  a higher ET cancellation rate was observed in the whole blastocyst-stage group;  the ET cancellation rate was reduced significantly if the decision to proceed to blastocyst transfer was made on day 3 after oocyte retrieval, which is a post hoc conclusion.