Although hepatitis C virus is a major cause of hepatitis, additional etiologic agent(s) continue to be sought. In 1995, hepatitis G, a newly discovered agent, was identified in sera from two different patients. Hepatitis G is an RNA virus similar to hepatitis C and mem-bers of the flavivirus family. An antibody assay can detect past, but not present, infection, and detection of acute infection with hepatitis G requires a PCR assay for viral RNA in serum. Up to 2% of volunteer blood donors are seropositive for hepatitis G RNA, which is a blood-borne virus. In addition to being closely related to hepatitis C, data suggest that the majority of patients infected by hepatitis C are also infected by hepatitis G. Given this association, it has been difficult to ascertain the contribution of hepatitis G to clinical dis-ease. Patients infected with both viruses do not appear to have worse disease than those infected by hepatitis C virus only. Currently, there is no useful serologic test and no ther-apy is established.