The present report summarizes the experience on the mar(X) syndrome in a total of 157 male patients (44 prepubertal and 113 postpubertal) ascertained through 83 index patients from 83 families under investigation. In one third of the families pedigree data were consistent with X-linked recessive inheritance. In the further two thirds of the families the presenting symptom was familial mental retardation with a mentally retarded mother, or mental subnormality with hyperkinetic behaviour in the male patient. No more than 60% of the adult males presented the typical clinical triad (mental retardation-long face-megalotestes). The most characteristic finding in the mar(X) boy is the psychological profile with severe hyperkinetism, hypersensitivity, handbiting and autistic features in some of them. In 4 of the 27 large mar(X) pedigrees strong evidence was present of a possible transmission of the mar(X) through normal males. The high incidence of mental subnormality in the female offspring of heterozygote carriers, and the relationship between mental status, phenotype, age and expression of the mar(X) in different culture conditions is discussed.