OBJECTIVES: Evidence has accumulated that an inflammatory, possibly autoimmune-mediated, process in the central nervous system (CNS), and by way of an aberrant immune system, may underlie the development of schizophrenia. Therefore, the aim of this study was to evaluate patients with schizophrenia or related psychosis for blood-brain barrier (BBB) function and immunoglobulin (Ig)G synthesis within the CNS. METHODS: Fifteen patients with schizophrenia or schizoaffective disorder and 12 controls were investigated using lumbar puncture and blood sampling. Cerebrospinal fluid (CSF) and serum/plasma (S/P) were analysed for albumin and IgG by standard laboratory methods, and the ratio of CSF-albumin to P-albumin (marker of BBB function) and the IgG index (marker of CNS IgG synthesis) were calculated. Additionally, the patients were assessed for clinical symptoms with the Positive and Negative Syndrome Scale for schizophrenia. RESULTS: The ratio of CSF-albumin to P-albumin was higher and the IgG index was lower in patients than in controls (p=0.045 and p=0.001, respectively). Moreover, subgroup analyses showed that patients in partial symptom remission had higher ratios of CSF-albumin to P-albumin than patients in full symptom remission, and that patients with heredity for schizophrenia or related psychosis had lower IgG indices than patients without heredity. CONCLUSIONS: In this study we show that patients with schizophrenia or related psychosis have impaired BBB function and lower IgG synthesis within the CNS, compared to controls. These findings support the view that a pathological process within the CNS, combined with an aberrant immune system, may underlie the development of schizophrenia.