Aim. The aim of the research was to determine the prevalence of diabetes mellitus type II in people suffering from mental diseases (schizophrenia and disorders similar to schizophrenia, affective disorders, anxiety disorders, the disease of addiction), determine if there is a correlation between the diabetes and body weight of patients, determine if there is a correlation between type of psychopharmacological drugs they were taking and diabetes, determine if there is correlation between duration of mental disorder and diabetes and determine if there is correlation between diabetes in patients suffering from mental disease and information about the presence of diabetes in relatives. Study outline. Cross-sectional study was conducted at the Department of Psychiatry, University Hospital Centre Osijek. Patients and methods. The study included 1264 patients suffering from mental diseases (schizophrenia and disorders similar to schizophrenia, affective disorders, the disease of addiction, anxiety disorders) treated at the Department of Psychiatry, University Hospital Centre Osijek 2010 – 2015. Medical history of the patients was used as a source of the data. Results. Diabetes is most commonly found in the schizophrenic patients, and it is most common in patients who have used clozapine as their primary therapy. Diabetes in people who used risperidone was significantly lower than expected. Conclusion. Diabetes mellitus type II is common in psychiatric patients, in the group of patients with schizophrenia respectfully. The most diabetes patients are treated with clozapine.