Chromosome abnormalities are observed very frequently in humans. Several types of structural chromosome abnormalities have been identified, with chromosome translocations, both reciprocal and Robertsonian, being the most common in the population. Balanced carriers of such rearrangements could be at risk of generating abnormal offspring due to the meiotic segregation of the translocation. Preimplantation Genetic Diagnosis (PGD) has allowed the extensive cytogenetic investigation of embryos from such patients with the application of Fluorescent in situ hybridisation (FISH). The first part of this work involved the development of robust three-colour FISH protocols for their clinical application for the PGD for three reciprocal translocations, two different Robertsonian translocations and two cases of suspected gonadal mosaicism. Five of these patients underwent 1-2 cycles of treatment, and 21 normal/balanced embryos were detected and transferred to the maternal uterus. One clinical pregnancy was established with a subsequent live birth of a healthy male infant in a case of a female reciprocal translocation carrier. Extensive FISH examination of the non-transferred embryos showed evidence of post-zygotic mosaicism in 73.4% of them, with chaotic embryos predominating. Both meiotic and mitotic mechanisms leading to chromosome gain and/or loss were identified in this group of embryos.