Previous studies have supported an association between low levels folate, vitamin B12 levels and elevated homocysteine levels as possible predictors of depression. Hyperhomocysteinemia induces free radical production, leading to alteration of oxidative lipid and protein modifications. Vitamin supplementation or antidepressants may reduce risk factors underlying depression. The aims of this study were to determine serum levels of protein carbonylation, lipid peroxidation, homocysteine, folic acid and vitamin B12 in depressive patients and to compare them with healthy controls; and to investigate the effects of sertraline (50 mg/day) treatment during 45 days. 23 depressed patients and 23 healthy controls participated in this study. Serum protein carbonylation was determined by spectrophotometric measurement of 2,4-dinitrophenylhydrason formation. Malondialdehyde levels were determined by spectrophotometric measurement of colour which was the reaction between thiobarbituric acid and malonclialdehyde. Serum homocysteine levels were measured by HPLC, and vitamin B12 and folate levels - by radioimmunoassay. There was no remarkable difference in protein carbonylation, malondialdehyde formation, homocysteine, vitamin B12 and folate levels between healthy control group and depressed patients. Sertraline treatment caused a significant decrease in malondialdehyde levels. The findings suggest that sertraline treatment caused decreasing in oxidative stress by lowering lipid peroxidation in depressed patients.