Mental retardation is a serious medical and social problem. The prevalence of mental retardation is estimated at 2-3%. Establishing the cause of mental retardation is extremely important for prognosis, management, and genetic counseling. It is postulated that 25-35% of mental retardation cases may be of genetic background. Among the genetic causes 25-30% are probably result of mutations located in the X chromosome (X-linked mental retardation--XLMR). X-linked mental retardation is a heterogeneous set of conditions responsible for a large proportion of inherited mental retardation. More than 200 XLMR conditions and 45 cloned genes are listed in catalogue available on the Internet. Traditionally, based on clinical presentation, XLMR conditions were divided into specific and nonspecific forms or syndromic and nonsyndromic. The distinction between specific and non-specific forms of XLMR is gradually becoming less clear and spectrum of phenotypic variability is very large as both syndromic and nonsyndromic forms have been described for several of the XLMR genes. Mutations in patients suffering from X-linked mental retardation genes have been found only in a relatively limited number of cases. Up to 50% of the patients from XLMR families might have mutations in one of the known genes implicated in XLMR so far. However, current methods are generally too expensive or too unreliable to justify mutation screening of all known XLMR genes in diagnostic testing. Thus it is necessary to use empirical data of recurrence risk in genetic counseling of the family with mental retardation.