Background/aims: Oxidative stress increases in chronic hepatitis C, and antioxidant defense mechanisms are impaired. The aims of this study were to compare chronic hepatitis C patients and healthy subjects according to oxidative stress and antioxidant system markers, and to determine the relationship between oxidative stress and hepatosteatosis. Methods: This is an observational study in a tertiary center. Twenty-nine biopsy-proven chronic hepatitis C patients, with no prior anti-viral treatment and persistently elevated serum transaminase levels for 6 months, were included. The control group included 46 healthy subjects. Advanced oxidation protein products and malonyldialdehyde levels were measured. Total radical-trapping antioxidant parameter was calculated. Results: Baseline characteristics were similar in the patient and control groups. In chronic hepatitis C patients, serum levels of advanced oxidation protein products were significantly higher than in the control group (235.0 +/- 142.8 vs 116.7 +/- 79.5, p < 0.001). Serum levels of malonyldialdehyde were also significantly higher than in the control group (9.3 +/- 2.1 vs 6.5 +/- 1.1, p < 0.001). However, there was no significant difference in total radical-trapping antioxidant parameter. The total radical-trapping antioxidant parameter/advanced oxidation protein products index was significantly lower in chronic hepatitis C patients than in healthy controls (p < 0.05). There was no significant correlation between advanced oxidation protein products and malonyldialdehyde and hepatosteatosis. Conclusions: We conclude that oxidative stress occurs in chronic hepatitis C, and antioxidant defense mechanisms are inadequate. Serum levels of advanced oxidation protein products and malonyldialdehyde are higher in chronic hepatitis C patients when compared to healthy individuals, and may be useful markers in chronic hepatitis C.