Although many countries' policies give Severe Mental Illness (SMI) priority inside Mental Health Services, researches assessing the prevalence of SMI in Mental Health Services according to operational criteria are still few. The aim of this is paper is to define annual SMI treated prevalence, describing socio-demographic and clinical characteristics, patterns of care and treatment costs of SMI and non-SMI patients. SMI prevalence in 10 Departments of Mental Health of the Lombardy Region (Italy) was assessed in 2000 by applying criteria concerning both severity, measured through HoNOS (Health of the Nation Outcome Scales), and utilisation patterns in the previous year. Annual SMI prevalence was equal to 3.1 cases per 1,000 inhabitants aged over 14; SMI patients' costs were 5.5 times higher than those of non-SMI patients ($5,183 versus $939 per year) and patterns of care were different. The variables predicting the SMI status were diagnosis, presence of paid employment, duration of service contact, care packages delivered in 2000 and severity of some HoNOS items (self harm, drug abuse, cognitive problems, delusions, other symptoms, relationships, activities of daily living and housing). The use of the combined criteria of severity and intensity of contact with mental health services in the previous year seems to be able to define severely ill patients adequately.