OBJECTIVE: To establish whether robust behavioural 'syndromes' can be identified from among the widely heterogeneous behavioural changes which occur in dementia. DESIGN: Longitudinal, prospective study with follow-up at 4 and 8 months. SETTING: Community settings in Oxfordshire, UK. PARTICIPANTS: 97 elderly people with a diagnosis of Alzheimer's disease or vascular dementia (in many cases confirmed by postmortem examination) and who were living at home with a carer. MEASURES: Each subject's behaviour was assessed in detail at each interview using the Present Behavioural Examination to assess subject's behaviour over the preceding 4 weeks. Seventeen key behaviour items which were both common and clinically important were selected for further analysis. RESULTS: Three syndromes were identified: (a) overactivity (walking more, walking aimlessly, trailing the carer or checking where the carer was); (b) aggressive behaviour (physical aggression, aggressive resistance, verbal aggression); (c) psychosis (anxiety, persecutory ideas and hallucinations). The same syndromes were found using data collected at three different time points and by using a variety of statistical techniques, confirming their robustness. CONCLUSIONS: Overactivity, aggressive behaviour and psychosis form three distinct behavioural syndromes in dementia.