This study investigated the frequency and severity of challenging behaviour in adults with Down's syndrome with and without signs of dementia. Care staff were interviewed using the Aberrant Behaviour Checklist-Community version (M.G. Aman & N.N. Singh, Slosson, East Aurora, NY, 1994), to investigate the frequency and severity of challenging behaviour. Individuals' 'dementia status' was assessed by using the Dementia Scale for Down's syndrome (Gedye Research and Consulting, Vancouver, 1995). 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.