Although mosaicism was shown to be a normal feature in cleaving embryos, its consequences for the late preimplantation stages are unknown. We performed blastocyst immunosurgery, followed by fluorescent in-situ hybridization (FISH), to determine the number of cells and degree of mosaicism in the inner cell mass (ICM) of human blastocysts. Of 47 ICM samples analysed, 20 had aneuploid cells, and two also had a few tetraploid cells. The average degree of aneuploidy in the ICM was similar to the overall blastocyst mosaicism, suggesting that there is probably no selection for euploid ICM. The lower degree of blastocyst mosaicism, compared with the cleavage-stage embryos, may be due to a mechanism of selection against the embryos with high frequency of mosaicism, leading to elimination of these embryos prior to blastocyst formation.