Human early cleavage stage embryos which survive cryopreservation and thawing fully intact demonstrate similar developmental potential to equivalent non frozen embryos when returned to the in vivo environment, whereas blastomere loss is directly related to the loss of potential for subsequent implantation in thawed embryos. This suggests that blastomere lysis during freezing and thawing does not occur preferentially in non viable blastomeres. Prefreeze growth rate rather than prefreeze blastomere number per se correlates with the developmental potential of stored embryos. When blastomere loss occurs as a consequence of cryopreservation, development of thawed early cleavage stage embryos to the blastocyst stage in vitro is impaired and the resultant blastocysts have a reduced total cell content. Blastomere loss is more prevalent in embryos which have been biopsied for preimplantation genetic diagnosis but this increased sensitivity can be circumvented by modification of the standard cryopreservation protocol.