This is a retrospective study aiming to assess telomere length inhuman embryos 4 days post fertilization and to determine whetherit is correlated to chromosomal ploidy, embryo developmental rateand patient age. Embryos were donated from patients undergoing treatmentin the assisted conception unit. Seven couples took part, generating35 embryos consisting of 1130 cells. Quantitative fluorescent in-situhybridization (FISH) measured the telomere length of every cell usinga pan-telomeric probe. Conventional FISH on six chromosomes was usedto assess aneuploidy in the same cells. Maternal and paternal age,referral reason, embryo developmental rate and type of chromosomalerror were taken into account. Chromosomally abnormal cells wereassociated with shorter telomeres than normal cells for embryos thatwere developmentally slow. Cells produced by women of advanced maternalage and those with a history of repeated miscarriage tended to havesubstantially shorter telomeres. There was no significant differencein telomere length with respect to the rate of embryo development5 days post fertilization. Telomeres play an important role in celldivision and shorter telomeres may affect embryonic ploidy. Reducedtelomere length was associated with aneuploid cells and embryos fromwomen of advanced maternal age.