The modes of transmission of GB virus-C/hepatitis G virus (GBV-C/HGV) other than by blood transfusion are largely unknown. The prevalence of GBV-C/HGV viremia and the associated risk factors in 145 female prostitutes were examined. The seroprevalence of hepatitis B surface antigen (HBsAg), antibodies to hepatitis C virus (anti-HCV), and GBV-C/HGV RNA were 14%, 18%, and 11%, respectively. The demographic characteristics were similar between subjects with and without HBsAg. In contrast, those with HCV or GBV-C/HGV infection had practised longer as prostitutes and received blood transfusion more frequently. Moreover, the prevalence of GBV-C/HGV RNA and anti-HCV tended to increase in parallel with the duration of prostitution. These results suggest that like HCV, sexual transmission of GBV-C/HGV occurs and the risk increased with prolonged duration of exposure. The transmission efficiency between GBV-C/HGV and HCV appears to be similar.