The fragile X syndrome (FRAX) is the most common familial form of mental retardation. The incidence is estimated at 1 in 4000 males. The leading symptom of the syndrome is mental retardation, with accompanying behavioural problems. About 25-35% of affected persons meet the criteria of autism. The behavioural problems involve attention problems, hyperactivity, tactile defensiveness, speech problems (echolalia), aggression, emotional problems, depression, anxiety, and stereotypic movements. There is no causal treatment and management is mainly symptomatic. Many specialists should be involved in this process. Behavioural and educational therapy is indicated. The basic step is identification of the child's problems. Its goal is to promote development of the child's abilities. Pharmacological intervention should be accompanied by supporting methods. The diagnosis of FRAX involves the entire family. The family members should be offered genetic counselling and the possibility of diagnostic DNA analysis.