OBJECTIVE: The aim of this survey was to estimate the prevalence of severe mental disorders in a representative sample of sentenced prisoners. METHOD: The subjects were selected as a random sample of sentenced prisoners in Melbourne's three metropolitan prisons. Interviews were conducted with 158 men and 31 women. Clinicians used the Structured Clinical Interview for DSM-III-R (SCID) to diagnose psychotic, affective, and substance use disorders. RESULTS: Six prisoners (3%) received current diagnoses of psychotic disorders, and 23 (12%) were diagnosed as having current mood disorders, mainly major depression. A lifetime diagnosis of at least one mental disorder each was made for 82% of the respondents, and in 26% more than one lifetime disorder was diagnosed. Sixty-nine percent received lifetime diagnoses of dependence on or abuse of alcohol, other psychoactive substances, or a combination of these. CONCLUSIONS: These findings do not indicate a large-scale shift of deinstitutionalized psychotically ill people from mental hospitals to prisons. They do, however, highlight the diversion into the corrections system of substance-dependent people and the apparent pool of prisoners with largely untreated major depression.