Fragile X syndrome is the most common monogenic cause of mental retardation in boys. It is always characterized clinically by moderate mental retardation and often by a long face with large everted ears and macro-orchidism. The causal mutation is an expansion of a CGG triplet repeat in a 5' exon of the FMR-1 gene in Xq27.3. We report here for the first time a method for preimplantation genetic diagnosis (PGD) for fragile X syndrome based on the amplification of the CGG triplet in the normal allele. Our candidate-patient population, as well as two clinical preimplantation genetic diagnosis (PGD) cycles which led to a pregnancy with an unaffected fetus, are presented in this paper.