Background: There is little consistency in how severe mental illness (SMI) is defined in practice, and no operational definitions.
Aims: To test two operationalized definitions, based on the National Institute of Mental Health (1987) definition: the first uses three criteria (diagnosis of psychosis; duration of service contact > or = 2 years; GAF score < or = 50), the second only the last two.
Method: Annual prevalence rates of SMI in two European catchment areas for each criterion and the criteria combined were calculated.
Results: The first definition produced rates of 2.55 and 1.34/1000 in London and Verona, respectively; the second permitted an additional 0.98/1000 non-psychotic disorders to be included in Verona.
Conclusions: The three-dimensional definition selects a small group of patients with SMI who have psychotic disorders. The two-dimensional approach allows estimates of SMI prevalence rates which include all forms of mental disorder.