Objective: Neuronal damage caused by free radicals is believed to be effective in pathogenesis of several psychiatric disorders. This belief is due to the toxic effects of free radicals that play a role in oxidative stress. Considering that the brain is one of the most sensitive organs to the oxidative damage, the importance of oxidative stress in psychiatric disorders will become more apparent. Additionally, high oxygen use in the brain, and its structure rich in lipid, which is one of the most sensitive molecules to the free radical damage, and its having the average antioxidant system yield support oxidative stress theory in the pathogenesis of psychiatric disorders. This study aimed to determine xanthine oxidase (XO), adenosine deaminase (ADA) and vitamins E levels in patients with schizophrenia and control groups, and to investigate the relationship between schizophrenia and the parameters by comparing the measured parameters with each other. Methods: Our study sample included 30 patients diagnosed with schizophrenia. The control group consisted of 30 healthy volunteers matched by sex with similar age and smoking habits. In the patient group and the control group, adenosine deaminase, xanthine oxidase, and vitamin E were measured manually using spectrophotometric methods. Results: Serum xanthine oxidase levels in the schizophrenic group were significantly higher than the control group levels. Serum vitamin E and adenosine deaminase levels in the schizophrenia group were significantly lower than the levels of the control group. Discussion and Conclusion: The fact that mechanism of schizophrenia pathogenesis which has a wide variety of clinical symptoms and a disease process is yet to be elucidated reveals the importance of this kind of studies. In this study, low levels of antioxidant vitamin E and adenosine deaminase, and high levels of xanthine oxidase suggest that oxidative stressmediated neuronal damage may play a role in the pathogenesis of schizophrenia. Therefore, we believe that further research with larger sample groups should be conducted.