The aims of this study were to determine the prevalence of severe mental illness (SMI) in patients in contact with mental health services and to determine the factors associated with SMI. A total of 260 patients who met diagnostic criteria for SMI were assessed using the Global Assessment of Functioning (GAF) scale and Health of the Nation Outcome Scales. The overall prevalence of SMI was 6.08 per thousand. According to the three different cutoff points with GAF, the prevalence of SMI ranged from 5.38 per thousand under the weak criterion (GAF < 70) to 1.01 per thousand under the strict criterion (GAF < 50). In the regression model, the dependent variable (presence of SMI) was defined using a GAF < 60, and the variables independently associated with the dependent variable were years of disease duration since diagnose, mental health service use, alcohol or other substance abuse, and depressive anxiety and other psychological symptoms.