This paper discusses the development and application of a socio-economic status (SES) index, created to explore the relationship between socio-economic variables and psychiatric service use. The study was conducted in a community-based mental health service (CMHS) in Verona, Northern Italy, utilising service use data from 1996. An ecological SES index was constructed through a factor analysis from 1991 Census data, at census block level. Three factors reflected the following domains: the educational-employment sector (with four components), the relational network (with three components) and the material conditions (with three components). All service users were assigned a SES value, according to their place of residence in 1996. When these data were explored spatially, using ArcView 8.3, an association was observed between socio-economic deprivation and psychiatric service use. The SES index was then successfully validated using occupational status at the individual level. This study confirms the usefulness of developing and validating an ecological census-based SES index, for service planning and resource allocation in an area with a community-based system of mental health care.