In an unselected series of mentally retarded children in the county of Västerbotten, Sweden, the annual incidence of children with severe mental retardation (SMR) (IQ &lt; 50) and alive at the age of one year decreased from 5.3 per 1,000 in 1959 - 1963 to 3.1 per 1,000 in 1967 -1970. This was mainly due to a decrease in the incidence of Down's syndrome. In parallel the proportion of mothers 35 years of age or more at the birth of the child decreased significantly. The prevalence of children with SMR in 1976 was 3.5 per 1,000. The main cause of the SMR was prenatal in 70 percent, perinatal in 8 percent and postnatal in 1 percent. The cause of the SMR was untraceable in 20 percent of the cases. Associated CNS-handicaps occurred in 52 percent of the cases. The annual incidence of mildly mentally retarded children (IQ 50 - 69) registered at the Bureau for Provision and Services for Mentally Retarded was 4.2 per 1,000 and the prevalence in 1979 was 3.8 per 1,000. The cause of the mild mental retardation (MMR) was untraceable in 43 percent. Prenatal causes were identified in as many as 43 percent. Perinatal causes were found in 7 percent and postnatal causes in 5 percent of the cases. Associated CNS-handicaps occurred in 30 percent of the cases.A syndrome of mental retardation with X-linked inheritance not recognized previously in Sweden was characterized clinically (mainly in boys, machro-orchidism, verbal disabilities) and cytogenetically (a fragile site on the X-chromosomes seen after cui turing in special folic acid deficient media) and found to have a prevalence of 6 percent in the population of severely mentally retarded boys. This makes this syndrome the next most common cause of SMR in boys after Down's syndrome. The chromosomal fragility was also identified in female carriers, which has implications for genetic counselling.Through identification of an untreated Phenylketonurie mother giving birth to five severely mentally retarded children, attention was focused on the risks for the fetus of the growing number of Phenylketonurie women identified neonatally and treated di etarily but untreated after the age of 10 - 15 years.Great improvement in intellectual and social ability was seen in a boy with phenylketonuria although the dietary treatment was not introduced until the age of eight years.Heavy irradiation of a fetus late in gestation caused mental retardation, microcephaly, stunted growth, and eye and teeth abnormalities, although such abnormalities are thought not to result from irradiation after 20 weeks of pregnancy. / <p>Endast s.1-71: sammanfattningen (kappan) i fulltexten.</p><p>Ej med i fulltexten s.72-145: 7 delar.</p> / digitalisering@umu