In the past decade it became accepted that free radicals, lipid peroxidation and antioxidant defense play a role in various tissues damages, thus in certain liver diseases as well. Since only limited data have been reported concerning the oxidative stress in viral hepatitis, a comparative study was performed in patients (pts) with chronic hepatitis C and alcoholic liver disease. In addition, the effects of a flavonolignan drug silymarin were assessed. 10 pts with chronic hepatitis C, 5 pts with alcoholic hepatitis and 13 pts with alcoholic cirrhosis have been investigated. Biochemical liver tests (serum bilirubin, aminotransferases, ALT, AST, lactate dehydrogenase (LDH), pseudocholinesterase, prothrombin), malandialdehyde (MDA) levels in plasma and red blood cell (RBC) hemolysate, superoxide radical generating capacity of stimulated polymorphonuclear granulocytes (PMN), plasma concentrations of reduced (GSH) and oxidized (GSSG) glutathione, vitamin A, luteine and beta carotene, furthermore RBC superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase activities were determined. The level of plasma MDA--as the marker of lipid peroxidation--was highest in alcoholic cirrhosis (five times of normal) (p < 0.05), the RBC hemolysate MDA was most elevated in chronic hepatitis C (p < 0.05). The mean PMNs' superoxide radical generating capacity was 116.6% of normal control in alcoholic hepatitis, where the mean GSH level was the lowest (89.8% of normal). Plasma vitamin A content was lowest in alcoholic cirrhosis (68% of control) (p < 0.05). SOD activity was elevated in both chronic hepatitis C and alcoholic cirrhosis, where GPx activity was decreased (p < 0.05). There was a correlation between LDH and SOD activities (r = 0.77, p = 0.015). Silymarin treatment of one month duration resulted in normalization of serum bilirubin in 55% of treated pts, AST became normal in 45%, and RBC hemolyzate MDA level normalized in similar rate. A significant increase in both GSH and retinoids was found. Alterations in oxidative stress and antioxidant defense system were shown in chronic hepatitis C, not only in alcoholic liver disease. The parameters of lipid peroxidation and antioxidant defense may be useful surrogate markers for monitoring pts with liver disease during hepatoprotective treatment.