Preimplantation genetic diagnosis (PGD) has been carried out for two couples with different mosaic Robertsonian translocations. Two PGD cycles for a mosaic 13;13 homologous Robertsonian translocation carrier resulted in the birth of a healthy child in each cycle, illustrating the importance of scanning G-banded preparations from homologous Robertsonian carriers for the presence of a normal cell line. One couple was referred for PGD because the male partner carried a mosaic 14;15 Robertsonian translocation with a normal cell line. A single PGD cycle resulted in the birth of a healthy child. Follow-up studies and extended FISH analysis of the carrier's lymphocytes detected three cell lines, two carrying different 14;15 Robertsonian chromosomes and one normal cell line. The two 14;15 Robertsonian chromosomes had different breakpoints in the proximal short arm regions. We suggest that the presence of the D15Z1 polymorphism on the short arm of one chromosome 14 mediated the post-zygotic formation of the two different Robertsonian chromosomes. © 2013 Wiley Periodicals, Inc.