Background: Previous research has suggested a variety of possible relationships between the presence of symptoms of psychiatric disorder and challenging behaviours in people with intellectual disability. This study explores this relationship in a total population sample of adults with challenging behaviour. Materials and Methods: Over 800 service settings in a defined geographical area were screened to identify individuals with challenging behaviour. Detailed behavioural data, Psychiatric Assessment Schedule for Adults with a Developmental Disability (PAS-ADD) checklist and Adaptive Behaviour Scale (Part 1) scores were collected on 76% of the 930 adults identified. Results: Just under 17% of participants reached threshold scores on one on more subscales of the PAS-ADD checklist. There was some evidence of increasing behavioural severity being associated with increasing psychiatric symptoms. There were no associations between specific forms of challenging behaviour and individual symptoms. Conclusions: The data would appear supportive of previous suggestions that it is unlikely that the majority of challenging behaviours in adults with intellectual disability are underpinned by psychiatric disorder.