698 children below age sixteen years with severe mental handicap (below IQ50) who were admitted to hospital between 1956 and 1959 were surveyed for genetic prognosis. They were divided into 23.7% ascribed primarily to environmental factors, 50.9% unclassified aetiologically, 16.2% of Down's syndrome, 5.3% other genetic syndromes and 3.9% with congenital hydrocephalus. The incidence of a similar degree of mental retardation among the sibs of 660 whose families were traced, was 1.1% in the "environmental" group; 4.7% for the unclassified; 1.7% for Down's syndrome, 11.5% for other genetic syndromes and 4.3% for hydrocephalus with spina bifida. There were no affected sibs of the uncomplicated cases of congenital hydrocephalus. This survey underlines the fact that a high proportion of cases of mental handicap remain without aetiological diagnosis. It emphasises the value of such a diagnosis for genetic counselling. Advice given depends on the circumstances of the particular case which requires a detailed social history and sympathetic rapport by members of the team as well as appropriate clinical expertise (Kirman, 1971, 1972). When families with similarly affected new relative or previous severely mentally handicapped sibs are set aside, the recurrent risk for unclassified cases is reduced to three per cent.