Introduction: In recent times schizophrenia has shown positive association with diabetes mellitus. Previous studies have suggested possible role of oxidative stress in pathophysiology of neuropsychiatric disorders. In the current work, we have measured oxidative damage to biomolecules in schizophrenia patients with and without diabetes mellitus to know the strength of association of schizophrenia with diabetes mellitus. Materials and methods: Serum samples from 39 patients having schizophrenia without diabetes mellitus (group I) and 21 patients having schizophrenia with diabetes mellitus (group II) and 50 healthy controls were collected to analyze lipid peroxidation marker malondialdehyde (MDA) and major antioxidant total thiol levels using colorimetric methods. Fasting blood glucose (FBG), serum urea, creatinine, aspartate transaminase (AST), alanine transaminase (ALT), total bilirubin and direct bilirubin were determined by automated analyzer Hitachi 912. Results: There was significant increase in FBG (P<0.001) and MDA (<0.001), and decrease in total thiols (p<0.001) in group I compared to healthy controls. There was significant increase in (FBG) (p<0.001) and MDA (p<0.001), decrease in total thiols (p<0.001) in group II compared to group I and healthy controls. However, there was no significant difference in serum urea, creatinine, AST, ALT, total bilirubin and direct bilirubin between patients and healthy controls. Serum MDA levels correlated negatively with total thiol levels (r = -0.275, p<0.01) and positively with FBG (r = 0.823, p<0.01) in group II patients. Conclusion: Our study has shown presence of oxidative stress in schizophrenia which is further enhanced in schizophrenia associated with diabetes mellitus.