Objective To explore the clinical significances of total antioxidation (TAO) and nitric oxide synthase (NOS) expression in patients with liver diseases.Methods The concentrations of TAO,NOS and nitric oxide(NO) were determined and the mechanism of their changes were analyzed in sera of patients with acute hepatitis(AH),chronic hepatitis(CH),liver cirrhosis(LC) and hepatocellular carcinoma(HCC).Results The abnormal rate of TAO was 80% in AH or in CH,and 50% in LC or in HCC,respectively.The sera NOS activity over normal reference value was 70% in patients with liver diseases.The abnormal rate of NO level was 70% in AH,CH and LC groups,and 48% in HCC,respectively.The average level of serum TAO was significantly higher in AH group or in CH group,but not in both HCC and LC groups than that in normal subjects.The average levels of NO and NOS were significantly higher in patients with liver diseases than those in normal subjects.However,the two markers were lower in HCC patients than those in AH,CH and LC groups,respectively.Conclusions The data suggest that the activity of NOS is close relation to serum NO level in patients with liver diseases,the increase of NO concentration may play a role in protection of hepatocytes.