Mental retardation is a serious medical and social problem. The prevalence of mental retardation in Western countries is estimated to be between 2 and 3%. Establishing the cause of mental retardation is essential for prognosis, management, and genetic counseling. It is estimated that 25-35% of mental retardation might have a genetic background. Of these genetic causes, 25-30% are probably due to mutations on the X chromosome (X-linked mental retardation, XLMR). XLMR is a heterogeneous set of conditions involved in a large proportion of inherited mental retardation. More than 200 XLMR conditions have been reported and 76 genes has been linked to them. XLMR conditions are commonly subdivided into syndromic and nonsyndromic forms on the basis of clinical presentation. The distinction between these forms of XLMR is gradually becoming less clear as phenotypes are described for several of the genes. The spectrum of phenotypic variability in XLMR is so large that mutations in several XLMR genes have been found in both syndromic and nonsyndromic (XLMR) pedigrees. About 42% of patients from families with an XLMR history might have mutations in one of the known genes implicated in XLMR. However, in genetic counseling we have to use empiric recurrence risk.