There is increasing evidence to support the high prevalence of mental illness in adults with intellectual disability and some studies suggest that the prevalence of mental illness may be higher than that of the general population. It is not uncommon for adults with intellectual disability to be referred to local community mental health centres or psychiatric hospitals because of the presence of challenging behaviours, such as aggression or criminal offending behaviour. Despite the emerging

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... rch in the area of diagnosis and treatment of mental illness in adults with intellectual disability, many are often not provided with appropriate and adequate mental health services. Two case studies illustrate the difficulties in accessing mental health services for this 'invisible' group of individuals with intellectual disability, and demonstrate the gaps in service delivery and clinical practice. The paper argues that mental health concerns should be considered when challenging behaviours are present in persons with intellectual disability. There is an urgent need for mental health services to re-examine their understanding of and clinical practice with adults with intellectual disability and mental illness.