In the introduction, a brief overview is given on the clinical aspects and classification of non-A, non-B hepatitis and on the discovery of hepatitis C virus. Using a recombinant hepatitis C virus antigen in an ELISA test system, for the demonstration of the antibody to hepatitis C virus, a new possibility became available in the serological diagnosis of non-A, non-B hepatitis. According to the previous hepatitis C virus antibody studies performed in Western Europe and in Hungary, the results of the present multicentre study also show that hepatitis C virus infection is frequent mainly in the post-transfusion chronic non-A, non-B hepatitis (87%) and in haemophiliacs (86%). In addition, antibody to hepatitis C virus occurs in open-heart surgery (27%) as well as in haemodialysed patients (20%), furthermore, it can be found in other forms of liver disease of various aetiology, e. g. in chronic HBsAg-positive hepatitis (33%), autoimmune liver diseases (20%) and chronic alcoholic hepatitis (14%) as well. The parenterally transmitted non-A, non-B hepatitis, practically the hepatitis C virus-related liver disease often leads to cirrhosis or even carcinoma. As its treatment is unsolved question yet, the prevention seems to be of great importance, the preventive measures of post-transfusion hepatitis C virus infection is listed.