Background: Community mental health nurses (CMHNs) are the backbone of specialist mental health services in the UK. This study evaluated the effects of training CMHNs in a systematic psychosocial intervention (PSI), to help family carers manage behavioural changes in their relative with dementia. Method: One hundred and thirteen family carers received PSI support from a 'trained' (experimental), or a 'usual practice' (control) CMHN. Primary (day-to-day behaviour problems and management) and secondary (carer mood) outcome measures were obtained at baseline, 6, 12 and 18 months. CMHN training and supervision commenced prior to, and continued throughout, the 18-month study. Results: Patient cognition declined in both groups, but problem behaviour reduced in experimental group families. However this effect was associated with the practice of some, not all, CMHNs. Carer management and mood improved with PSI support. In contrast, by 18 months, families supported by conventional CMHNs reported reduced coping resources, increased problem behaviour and their level of depression worsened. Only two CMHNs adhered to the PSI-protocol. Conclusions: Some, but not all, 'trained' CMHNs used the PSI to help family carers reduce behavioural problems. A relatively long but not intensive PSI of 12-18 months was needed to moderate carer mood. Dementia-specific practice arrangements, training and sustained clinical supervision are important for the delivery of effective psychosocial interventions in dementia.