Fragile X syndrome is the most common form of inherited mental retardation affecting males. This disorder is a result of an expanded CGG trinucleotide repeat in the 5’ end of exon 1 of the FMR1 gene at Xq27.3. Here, we report on two mentally retarded boys suspected to be affected by X fragile syndrome and for who molecular analysis allowed us to confirm diagnosis in only one boy. Genetic exploration was based on molecular analysis of FMR1 gene by PCR and southern blotting. Molecular DNA tests showed a full mutation of FMR1 gene with a large expansion of 300 to 2000 CGG repeats in the 5’ untranslated region of FMR1 (Figure 1). The mother showed a premutation status with 100 CGG repeats. Mutated FMR1 alleles are detected in 47% of at risk individuals from familial cases but only in 6% of sporadic cases. However, a normal southern-blot pattern does not imply an absence of the syndrome. The molecular study of our family enabled us to analyse the expansion of the CGG repeats from one generation and to offer genetic counselling. The mother was a carrier of a large premutation which can be inherited from his father. She transmitted to her son the premutated allele even more expanded as a full mutation. Her daughters can inherit the premutation and transmit it as a full mutation to their sons. Since molecular diagnosis of FMR1 rearrangement become available, they can request prenatal diagnosis.