Psychotic syndromes such as schizophrenia are serious disorders with far-reaching effects on health, which have consequences for daily life of patients and their families. More than 1% of the population has a serious psychotic disorder, with often a first onset between 15 and 30 years of age. Treatment of psychotic disorders is still mainly symptomatic since the pathogenesis of these diseases is heterogeneous; genetic and environmental factors play a role that is still poorly understood. In a small subgroup of patients, there are strong indications that autoantibodies are responsible for causing a psychotic episode suggesting an autoimmune disease as the underlying cause. The aim of this study is to establish the prevalence of autoantibodies against neuronal surface proteins in an early onset psychotic patient’s cohort. This will improve the diagnosis of this specific patient group and would be an important achievement because, fortunately, a standard immunosuppressive treatment is available, which has been successfully applied in other autoimmune diseases. In this way, patients could benefit by standard immunosuppressive treatment in the clinic. Our study has been approved in 2016 by the METC of the MUMC+.