A 13-year incidence study of sex chromosome abnormalities in Arhus, Denmark of 34,910 newborn children showed that 1 per 448 had a sex chromosome abnormality. The incidences of the most common sex chromosome abnormalities were Klinefelter syndrome, 1 per 576 boys; XYY, 1 per 851 boys; triple X, 1 per 897 girls; Turner syndrome, 1 per 2130 girls. Follow-up of children with autosomal abnormalities is not included in this study. None of the 78 surviving children with sex chromosome abnormalities was mentally retarded. All children above school age attended regular schools. Seventy-seven percent of Klinefelter, triple X, and XYY children aged 15-19 had received remedial teaching, 29% were receiving remedial teaching at the last follow-up, 32% had been in special classes at a regular school due to learning problems, and 24% were still in such classes at the last time of follow-up. There was no increased frequency of criminal activity or behavior disorders, nor was there any increased frequency of mental or physical disorders. The distribution of planned training or occupation for the 25 youths with sex chromosome abnormalities between 15 and 19 years of age was similar to that of their sibs. Testosterone undecanoate treatment has been given to Klinefelter boys from puberty and growth hormone treatment to Turner girls from the age of 7, and very small doses of estrogen were given to these girls from around the age of 12 when FSH was increasing to postmenopausal levels. Prevention or reduction of deviations in mental development from the normal range in children with sex chromosome abnormalities is possible if educational and social resources are available and the parents are well informed and counseled regularly. Information in Denmark has been given in part by publishing four booklets about triple X, XYY, Turner, and Klinefelter syndrome. Information, support, and stimulation to self-help have, to a certain extent, been given through contact groups. Parents having a child with a sex chromosome abnormality need information, counseling, and assistance. The type and magnitude of this assistance depend on the individual child, the specific sex chromosome abnormality, and the parents' own resources, psychologically, socially, and otherwise.