Survey of adolescents with severe intellectual handicap. A diagnostic survey was undertaken of children aged 11 to 19 years in Tameside with severe learning difficulties (intelligence quotient less than or equal to 50). Eighty-two children were identified and their medical records reviewed. A specific diagnosis for the retardation was documented in 25 (30%) of the children, 18 of whom had Down's syndrome. A probable aetiology or a disorder of unknown aetiology had been identified in a further 21 (26%) children. To confirm the existing diagnosis, identify new diagnoses, and offer genetic counselling, the parents of 63 children were offered detailed reassessment of their child. Fifty three children were reviewed, and a specific disorder identified in 25 out of 31 previously undiagnosed children. The most frequent diagnoses made were fragile X syndrome and Rett's syndrome. On completion of the survey, 61 of the 82 children (74%) had a specific diagnosis or probable aetiology identified, 12 (15%) had associated disorders such as cerebral palsy, and in only nine of the 82 children (11%) were there no clues at all to the cause of their retardation. 