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Background: A high seroprevalence of Toxoplasma gondii infection has been detected in psychiatric patients, particularly in schizophrenia cases. Methods: In the present study 98 patients suffering from schizophrenia (58 inpatients and 40 first-episode patients) and 96 control patients (50 healthy volunteers and 46 with a depressive disorder) were examined for the presence of both IgG and IgM antibodies against T. gondii by enzyme-linked immunosorbent assay (ELISA). We applied the Bradford Hill criteria to identify the weight of causal inference. Results: The positivity rate of anti-T. gondii IgG antibodies among individuals with schizophrenia (57.1%) was significantly higher than in healthy controls (29.2%). There were no associations between immune status ratio (ISR) values and the risk of schizophrenia. The weight of evidence approach using the Bradford Hill criteria revealed a 92% probability of a causal association. Conclusion: Our results show that exposure to T. gondii may lead to schizophrenia. (C) 2010 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved.