Background: Approximately one-eighth of the population will have DSM-IV borderline intelligence. Various mental disorders and social disability are associated with it. Method: The paper uses data (secondary analysis) from a UK-wide cross-sectional survey of 8450 adults living in private households. Data were collected on psychiatric disorders, intellectual level, social functioning and service use. Results: In total, 12.3% of the sample had borderline intelligence. The prevalence of psychotic disorder was not significantly increased, but the group showed significant social disadvantage and increased rates of neurotic disorders, substance misuse and personality disorders when compared with their counterparts of normal intelligence. The borderline group was more likely to receive psychiatric medication, but not talking therapies. They appear to use significantly more services, including emergency services. Conclusion: Adults with borderline intelligence are more likely to suffer from treatable mental disorders and an excess of substance misuse. Services should be aware of hidden morbidity in this group.