The absence of reliable markers to identify viable embryos for transfer at the early cleavage stage may contribute to the generally low implantation rates and high incidence of multiple births associated with IVF treatment. In the present study, we examined the relationships among the timing of the first cleavage, incidence of blastocyst formation in vitro, and pregnancy outcomes. All embryos (n=1,748) were examined for early cleavage at 25 h after insemination. Three groups were defined based on the persistence of two pronuclei (PN group), pronuclei breakdown (BD group), and early cleaved zygotes (EC group). Each embryo (n=1,120) was cultured until day 6 of development, and assessed for blastocyst formation. The rate of blastocysts showing good morphology was significantly higher in the EC group (44.7%) than in the BD (26.9%) or PN (13.9%) groups. The cryothawed good morphology blastocysts were transferred in 209 cycles. The clinical pregnancy rate was the highest and the abortion rate was the lowest in the EC group. These results indicate that early cleavage is indicative of increased developmental potential in human embryos and may be useful as an additional criterion when selecting embryos for transfer.