The results showed that the dementia group displayed more frequent and severe forms of challenging behaviour than the nondementia group. The difference in reported levels of challenging behaviour of both groups with the general learning disabilities population was not considered to be clinically significant and levels fell predominantly within the 'normal range'. The findings of this study suggest that frequent and severe forms of challenging behaviour in adults with Down's syndrome is more likely to be a behavioural symptom associated with the onset of a dementing illness and not due to normal ageing alone.