Fragile X syndrome (FXS) is the most common X-linked mental retardation disorder. Most patients are males. In Thailand, the incidence of FXS in boys with mental retardation or delayed development is approximately 7%. Typical clinical characteristics in affected males include variable degrees of mental retardation, narrow and long face, large and prominent ears, enlarged testicles, hyperactivity, attention deficit, and autistic-like behaviors. It is caused by Cytosine-Guanine-Guanine (CGG) repeat expansion on the promoter region of the fragile X mental retardation-1 (FMR1) gene. Normal individuals have CGG repeats ranging from 6 to 55. In carriers, so called premutation, the CGG repeats range from 56 to 200 and expand to be more than 200 repeats for affected patients (designated as full mutation). Women with premutation stage can transmit the FMR1 gene with CGG repeat instability to her children with potential full mutation stage. Prenatal diagnosis and genetic counseling should be given to the family with this genetic disorder.