The Fragile X (FRAXA) syndrome is the most common cause of familial (monogenic) mental retardation and is widespread in human populations. This syndrome is characterised by an unusual mode of transmission for an X-linked disease. In affected families, one frequently finds clinically normal transmitting males, whose daughters - also clinically normal - have a high risk of having affected children. The risk of developing the disease (penetrance) thus appears to increase in successive generations of the same family through maternal transmission. As shown by molecular cloning of the fragile X locus, Fragile X mutations are unstable expansions of a CGG trinucleotide repeat, located in the first exon (non-protein-coding) of the FMR1 gene (for Fragile X Mental Retardation). Two main types of mutation are observed in affected families. A full mutation is found in patients with mental retardation and corresponds to large expansions of the repeat. Premutations are moderate expansions and are found in normal transmitting males and in the majority of clinically normal carrier females. About 15% of patients show a mosaic pattern consisting of both full mutations and premutations. Although analysis of the CGG expansion has led to the establishment of reliable tests for diagnosis and genetic counseling of Fragile X syndrome, care must be exercised to use these tools to answer the concerns of the families and avoid doing harm. In our opinion, testing in children should be restricted to those who show a developmental delay, cognitive deficits and/or abnormal behavior evocative of the syndrome. A carrier diagnosis in a girl who is clinically normal should probably only be performed at an age where she can understand the consequences for family planning and the options of prenatal diagnosis. When testing children with borderline cognitive deficits, a positive diagnosis should be used to improve educational strategies for the children - and not to stigmatise them.