OBJECTIVES: The origins of behavioural and psychological symptoms of dementia are still poorly understood. By focusing on piecemeal behaviours as opposed to more robust syndrome change valid biological correlates may be overlooked. Our understanding of BPSD via the identification of neuropsychiatric syndromes. METHODS: We recruited 435 subjects from old age psychiatry and elderly care memory outpatient clinics fulfilling the criteria for diagnosis of probable Alzheimer's disease. Behavioural and psychological symptoms were assessed using the Neuropsychiatric Inventory. Principal components factor analysis was carried out on the composite scores of the 12 symptom domains to identify behavioural syndromes (factors). Results were confirmed by performing three different rotations: Varimax, Equamax and Quartimax. RESULTS: Four factors were identified (which accounted for 57% of the variance): 'affect' factor-depression/dysphoria, anxiety, irritability/lability and agitation/aggression; 'physical behaviour' factor-apathy, aberrant motor behaviour, sleep disturbance and appetite/eating disturbance; 'psychosis' factor-delusions and hallucinations; 'hypomania' factor-disinhibition and elation/euphoria. These groups were unchanged when different methods of rotation were used. CONCLUSIONS: We report novel observations that agitation/aggression/irritability cluster within a depressive symptom factor and apathy is found within a physical behaviour factor. Copyright 2004 John Wiley & Sons, Ltd.