Analysis of chromosomes in human embryonic nuclei would ideally be achieved by karyotyping. Several studies have used this technique to examine human embryonic chromosomes (1–4), but information is limited, since it is difficult to obtain bandable metaphase spreads. For preimplantation genetic diagnosis (PGD) where only one or maybe two cells are available, karyotyping is not a viable option because of its low success rate per single cell. However, in certain cases, fluorescent in situ hybridization (FISH) can be used, for instance, for sexing embryos for couples at risk of passing on X-linked disorders for which there is, as yet, no specific molecular diagnoses. In addition FISH is useful for couples who are subfertile owing to chromosomal disorders (translocations or gonadal mosaicism). Since these are among the most common reasons for requesting PGD, a laboratory contemplating providing this service should include FISH in its repertoire