We studied the rate and pattern of recombinations within the extended major histocompatibility complex (MHC) locus of the human embryos obtained during preimplantation genetic diagnosis (PGD) for HLA compatibility. Recombinant allele frequency was on average 5.33 %, and recombination rate was 0.44 cM/Mb in the 12.2 Mb of the extended MHC locus. Recombination rate varied up to 14-fold (0.19-2.73 cM/Mb) between cases, and maternal recombination rate was on average 3.8 times higher than paternal alleles. More than 69 % of the recombination hot spots were clustered within the extended class II region where the recombination rate was 5.4 times more than that in extended class I region. These findings indicate the potential of PGD to study the mechanisms of linkage disequilibrium within MHC locus of human embryos, demonstrate the recombination characteristics within extended MHC loci of human embryos in comparison to sperm and family studies, and point to the significance of design and interpretation of PGD for HLA compatibility to avoid misdiagnosis because of meiotic recombinations.