In this study we describe the pre-clinical development and clinical application of preimplantation genetic diagnosis (PGD) by fluorescence in-situ hybridization (FISH) for two non-related carriers (one male and one female) of the most common balanced reciprocal translocation: t(11;22)(q25;q12). For the couple with the female carrier, enumeration of the sex chromosomes in the embryos was also indicated (husband: 47,XXY karyotype). Four-colour FISH analysis was performed on six blastomeres from three embryos. No embryo transfer was possible because all the embryos were unbalanced. Three PGD cycles, with two-colour FISH, were carried out for the couple with the male translocation carrier. A total of 35 embryos were biopsied and diagnosed by FISH; nine out of the 35 embryos (25. 7%) were normal and seven of them were transferred (two embryos from the first and four from the third cycle), six out of 35 embryos (17%) were unbalanced, three out of 35 embryos (5.7%) were triploid or polyploid, 10 out of 35 embryos (28.6%) were mosaic and seven out of 35 embryos (20%) were chaotic. Diagnosis failed in 2.9% of the embryos. The spermatozoa of the male carrier were also analysed using three-colour FISH. Only 29.1% of the sperm cells seemed to be balanced or normal. By choosing probes lying on both sides of the breakpoints and by using a combination of sub-telomeric or locus-specific probes and centromeric probes, the use of three-colour FISH enabled detection of all the imbalances in sperm and/or cleavage-stage embryos in the patients. This may improve risk assessment and genetic counselling in the future for translocation carriers.