Fluorescent in-situ hybridization (ISH) to interphase nuclei of human preimplantation embryos has been demonstrated with the X and Y chromosome-specific DNA probes, pBamX7 and pHY2.1, respectively. Assigning the sex on the basis of the number of hybridization signals in the majority of nuclei, the efficiencies with both probes to nuclei from male embryos were considerably higher than those previously reported for pHY2.1 detected by isotopic or conventional non-isotopic methods. Only approximately 15% of nuclei from male embryos failed to hybridize with these probes. With pBamX7, a high incidence (18%) of nuclei with two (or more) signals in embryos classified as males and four signals in a female embryo was observed. In some cases, the double spot nuclei were larger than those with single spots, providing evidence of tetraploidy. The feasibility of using fluorescent ISH for sexing biopsied embryos in couples at risk of X-linked disease and for the preimplantation diagnosis of chromosome abnormalities is discussed.