Fragile X syndrome is the most common familial form of mental retardation. The incidence is estimated to be 1 in 4000 male births. The disease is caused by amplification of trinucleotide repeats CGG in the first exon of FMR1 gene, located on the distal part of the long arm of the X chromosome. The main symptom of the disease is mental retardation, usually of moderate or profound degree. Characteristic clinical features of the disease observed in the affected person after puberty involve: an elongated face, large protruding ears and macroorchidism. Diagnosis is usually made late, when the child is 3-4 years old. Making diagnosis early is very difficult because of a lack of specific symptoms. We can observe developmental delay in children, with very late development of speech and behavioural problems with autistic features. Early diagnosis is very important, because its identification of high genetic family risk. The risk of recurrence for next children is as high as 50% and is stable for each pregnancy..