Background and Aims:Several studies have shown that the level of antibodies to Toxoplasma gondii (T. gondii) in schizophrenia is higher than in controls. And it is hypothesized that the neurotropic intracellular protozoan may contribute to the development of schizophrenia. The present study is to explore whether Bactrim, which is effective to T. gondii, can help to treat Toxoplasma-seropositive patients with schizophrenia.Methods:99 patients who were positive in IgG or IgM antibody to T. gondii were seperated into two groups randomly. One group recceived SMZco tablets (contained 400mg SMZ and 80mg TMP, 6 tablets/d) plus risperidone, and the other group received placebo and risperidone. Both the physicians and the patients did not know whether the patients took SMZco or placebo. The dosage of risperidone was tittered according to the patient's clinical symptoms and the maximum dose is 6mg/d. PANSS scales were used to assess the symptoms before and after treated for 6 weeks.Results:There was no significant difference in the scores of PANSS at baseline and after treatment. The rate of adverse effects, and the ratio of patients who have combined with benzodiazepine and anticholinergic medicine between the two groups did not differ significantly, while two patients in the SMZ group dropped out because of serious nausea and vomiting. The maximum dosage of risperidone in the SMZ group was lower than the placebo group (P<0.05).Conclusions:For toxoplasma-seropositive patients with schizophrenia, Bactrim may be helpful by decrease the dose of the antipsychotic medicine.