Excess embryos obtained from intracytoplasmic sperm injection cycles may be cultured and observed until day 5 if the couple receiving treatment do not want them to be cryopreserved. In order to investigate the correlation between blastocyst formation in extended culture and pregnancy outcome, 194 patients treated in two separate IVF units were examined retrospectively. The patients were separated into two groups: group 1 with at least one blastocyst formed in culture, and group 2 with no blastocyst formation. The pregnancy rates were 60.0% and 41.7% for groups 1 and 2, respectively. The pregnancy rate in group 1 was statistically significantly higher than in group 2 (P = 0.01). The results suggest that the developmental potential of embryos obtained from a single assisted reproduction treatment cycle may be similar and that blastocyst formation in vitro may help to predict the pregnancy outcome of that cycle. © 2008 Published by Reproductive Healthcare Ltd.