With the development of commercially available sequential media it is now possible to grow human embryos to the blastocyst stage without feeder cells. The transfer of blastocysts offers several advantages, the most important being synchronization of the embryos with the uterine endometrium and selection of the best quality embryos with a high implantation potential. This study was conducted to compare the efficiency of day 2 and day 5 transfer in a prospective randomized trial involving patients for whom embryo selection was possible (i.e. those with more than three embryos on day 2 following insemination). We obtained equivalent clinical pregnancy rates per cycle for day 2 (41.7%) and day 5 (38.8%) transfer, but fewer embryos were transferred on day 5 (2.24 versus 3.03). The implantation rates were 18.9% on day 2 and 24.1% on day 5. Selected patients with a good response to gonadotrophins (at least eight good quality metaphase II oocytes) may therefore benefit from blastocyst transfer by a reduction in the multiple pregnancy rate, provided no more than two (or even one) blastocyst is transferred.