The total number of adults with Down's syndrome living in Leicestershire, ascertained by widespread enquiry, was found to be 378. Of these, 371 were matched with adults with mental handicap due to other pathologies, on the basis of age, sex, and type of residence. Those with Down's syndrome were found to have a different spectrum of mental disorders from those without the syndrome. In particular, Down's syndrome patients were more likely to have been diagnosed as having depression and dementia; the controls were more likely to have been diagnosed as suffering from conduct disorder, personality disorder, or schizophrenia/paranoid state. The same proportion of each group had been given a diagnosis of autism.