There has recently been an expansion of paraprofessional staff involved in the provision of care for the severely mentally ill. In this paper we shall evaluate the effects of a community support worker (CSW) service in South London. A sample of patients with severe mental illness receiving the service was assessed over a 6-month period in order to examine the effects of continuing use of CSWs on outcomes, service use and costs. During the study period there were improvements in outcome reflecting service satisfaction, needs, quality of life and social behaviour. Whilst provision of the CSW service did not lead to extra total service costs, service use and cost data suggest that CSWs are associated with the substitution of in-patient use by community-based services.