Although infection with GB virus-C/hepatitis G virus (GBV-C/HGV) by blood transfusion is well documented, little is known about the other routes of transmission. The prevalence of GBV-C/HGV infection in spouses of index patients and the related risk factors were studied. Hepatitis C virus (HCV) and GBV-C/HGV infections were studied in spouses of 100 patients with hepatitis C, of whom 12 were found to be also positive for GBV-C/HGV RNA. For couples both with GBV-C/ HGV viremia, nucleotide sequences of the divergent envelope region were analyzed by phylogenetic tree constructions. For HCV infection, anti-HCV was found in 14 (14%) of the 100 spouses. Five spouses (42%) of the 12 patients with dual infection of GBV-C/HGV and HCV had evidence of GBV-C/HGV infection, three had viral RNA, and two had antibodies to a recombinant HGV envelope protein E2. Nucleotide sequence comparison and phylogenetic tree analysis of the genome in the GBV-C/HGV infected couple revealed the isolates to be closely related. These results suggest that spouses of patients with GBV-C/HGV infection are at a higher risk of acquiring GBV-C/HGV as compared with HCV, and they should be educated to avoid GBV-C/HGV infection from their spouses, in case GBV-C/HGV is shown to be pathogenic.