Some 20% of cases of posttransfusion and sporadic hepatitis non-A, non-B are anti-HCV negative. In 1995 it proved possible in collaboration of Genelabs with Boehringer Co. Mannheim to identify a new RNA virus which causes acute and chronic hepatitis in humans and tamarins. The genome of the virus contains some 2900 amino acids, and as to its structure, it resembles flaviviruses. It was described as hepatitis G virus (HGV). It differs from the hepatitis C, virus as it has only a 26% homology of amino acids. It is transmitted through blood during transfusion along with other parenteral routes of infection. Risk groups comprise i.v. drug addicts, blood donors and patients with thalassaemia and repeated blood transfusions. HGV can infect the liver as an independent virus or along with the virus of hepatitis B or C (dual infection). As to clinical aspects, hepatitis G is very mild and not associated with jaundice. Some patients develop chronic hepatitis. About half the patients infected with HGV have only a slightly raised transaminase activity, the remainder have normal liver enzymes. As compared with hepatitis C, the mean transaminase activity is one half. It can be diagnosed by assessment of HGV RNA by means of PCR. In the USA the prevalence of HGV RNA in blood donors with normal ALT activity is 1.7% and in donors with increased ALT activity 1.5%. The virus is sensitive to interferon, after treatment the serum concentration of HGV RNA declines rapidly but after withdrawal of treatment the values return to pre-treatment levels. This is the first report on the newly discovered hepatitis G virus.