An RNA virus, designated hepatitis G virus (HGV), was identified from the plasma of a patient with chronic hepatitis. Extension from an immunoreactive complementary DNA clone yielded the entire genome (9392 nucleotides) encoding a polyprotein of 2873 amino acids. The virus is closely related to GB virus C (GBV-C) and distantly related to hepatitis C virus, GBV-A, and GBV-B. HGV was associated with acute and chronic hepatitis. Persistent viremia was detected for up to 9 years in patients with hepatitis. The virus is transfusion-transmissible. It has a global distribution and is present within the volunteer blood donor population in the United States.

HGV was common in a group of volunteer blood donors, and it can be transmitted by transfusion. Most HGV infections were not associated with hepatitis. HGV did not worsen the course of concurrent HCV infection. No causal relation between HGV and hepatitis has been established.

Patients who only had HCV infection did not differ from patients with HCV and HGV co-infection in clinical presentation, HCV RNA level, or response of HCV to interferon-alpha therapy. Thus, HGV infection had no apparent influence on the clinical or virologic course of HCV infection. Hepatitis G virus was uniformly sensitive to interferon-alpha therapy, but only a few patients had a sustained virologic response.

HGV RNA was found in only a low percentage of patients with either acute or chronic non-A-E hepatitis: HGV appears to co-infect or superinfect in 10-15% of HCV infections and in 5-15% of HBV infections; the prevalence of HGV infection (0.8%) among voluntary blood donors in Japan is similar to that for HCV infection; a history of blood transfusion was obtained in 22 (55%) of the total 40 HGV-positive subjects; and isolated HGV infection appears to have a low disease burden.