Background Although several studies have mentioned associations between neuropsychiatric symptoms, there have been no prospective studies determining interrelations among behavioural sub-syndromes. Objectives To investigate the influence of several clinical variables on the course of neuropsychiatric symptoms, and to determine interrelationships between the behavioural sub-syndromes. Methods One hundred and ninety-nine patients with dementia were assessed every six months for two-years, using the Neuropsychiatric Inventory (NPI) to evaluate neuropsychiatric symptoms. Results Age, sex, and socioecononuc status were not associated with a specific neuropsychiatric symptom. Greater cognitive impairment was related to more severe psychosis, and dementia stage influenced the course of total NPI problems. There were strong interrelations among most behavioural sub-syndromes. The sub-syndrome hyperactivity was of influence on the development of psychosis, but not vice versa. Neither was the sub-syndrome mood/apathy of influence on the course of psychosis. Conclusions While different neuropsychiatric symptoms have their own specific correlates, there is a strong interrelationship between behavioural sub-syndromes. The data have implications for clinicians and the nosology of neuropsychiatric symptoms in dementia.