GB virus-C (GBV-C)/hepatitis G virus (HGV) is a positive, single-strand RNA virus that has been classified in the family Flaviviridae. Interestingly, GBV-C/HGV appears to have a truncated or absent core protein at the amino terminus of the polyprotein. GBV-C/HGV is transmitted parenterally and probably sexually. Most GBV-C/HGV infections appear to be asymptomatic, persistent, and no correlation between virus infection and liver dysfunction although the disease-inducing activity of GBV-C/HGV remains to be investigated. Furthermore, there was no evidence of pathogenesis in the liver by experiment with chimpanzees. From these results, GBV-C/HGV might be considered as a kind of "orphan" virus in search of a disease. Epidemiological investigation demonstrated that GBV-C/HGV infection is present in about 1-1.4% of the healthy population in developed countries and in 8-14.6% in developing countries. The genome of GBV-C/HGV exhibits a sequence variation among different isolates. On the basis of this variation, it has been proposed that GBV-C/HGV can be classified into at least four major genotypes, consisting of type 1 (West Africa), type 2 (US/Europe), type 3 (Asia), and type 4 (Southeast Asia).