AbstractAn etiological survey is presented of all severely retarded children living in Hertfordshire, at home and in residential care, born between January 1, 1965, and December 31, 1967. One hundred and forty‐six children (87 boys and 59 girls) were ascertained, out of a total population of 46,960, with a prevalence of 1 in 320 or 3.1 per 1,000. Approximately 1/3 (47) had the Down syndrome, 1 per 1,000 population.It was possible to establish a diagnosis in a further 45 cases, which included 1 additional case of autosomal chromosome abnormality and 7 each of autosomal dominant, recessive and X‐linked conditions; 17 were associated with presumed multifactorial etiological factors; in 6 the condition was thought to have been caused by an environmental agent. It was not possible to establish a cause in the remaining 54 cases.Recurrence risks of severe mental retardation in cases where it is impossible to establish a definite diagnosis are discussed and the potential value, for genetic counseling purposes, of categorizing such patients into broad symptomatological groups, is sugges