Background: Informant-based assessment of behavioural change and difference in dementia in Down syndrome can aid diagnosis and inform service delivery. To date few studies have examined the impact of different types of behavioural change. Methods: The Assessment for Adults with Developmental Disabilities (AADS), developed for this study, assesses behavioural excesses (11 items) and deficits (17 items) associated with dementia. Inter-informant reliability, internal consistency and concurrent validity were evaluated and found to be robust. Results: A comparison of the AADS subscale scores for three groups (n = 12) of adults with Down syndrome demonstrated more frequent deficits and excesses and greater management difficulty and effects on the individual in a dementia group than age comparable and younger groups. Conclusion: The AADS is a promising dementia specific measure for people with intellectual disability. Further research should evaluate change as dementia progresses and the nature of management difficulty and effects on the individual.