Objective. Non-alcoholic steatohepatitis ( NASH) is a form of chronic hepatitis. The pathogenesis of NASH has been dealt with in only a few studies and so it has not been clearly identified yet. The purpose of this study was to investigate the roles of TNF-alpha, TGF-beta, IL-6, IL-8, malondialdehyde ( MDA), nitric oxide ( NO) and the expression of Bcl-2 and Bax in the pathogenesis of NASH. Material and methods. The study included 92 patients, 57 of whom were diagnosed with biopsy-proven NASH, 13 with biopsy-proven hepatosteatosis and 22 with ultrasonography-diagnosed hepatosteatosis. Serum levels of TNF-alpha, TGF-beta, IL-6 and IL-8 were measured using the ELISA method. The plasma levels of NO were studied using the Griess method. Expressions of Bcl-2 and Bax were examined in paraffin blocks of liver biopsy materials by means of immunohistochemical-staining. MDA levels were measured using the thiobarbituric acid method. Results. No significant difference was found in the levels of TNF-alpha, TGF-beta, IL-6 or NO between the three groups ( p > 0.05). No difference was found in expression of Bcl-2 and expression of Bax between the biopsy-proven NASH and biopsy-proven hepatosteatosis groups ( p > 0.05). In the NASH group, the levels of IL-8 and MDA were found to be higher than those in the hepatosteatosis groups ( p < 0.05). Conclusions. The elevated levels of MDA may indicate the relationship between oxidative stress and NASH. Furthermore, IL-8 was found to be higher in the NASH group than in the hepatosteatosis group, demonstrating the importance of inflammation in the pathogenesis of NASH.