The prevalence of GB virus C (GBV-C)/ hepatitis G virus (HGV) RNA and antibodies to the structural E2 protein was investigated in a cohort of HIV-1 infected patients. Of 346 individuals, RNA was detected in 143 and E2 antibodies were detected in 73, for an overall prevalence of 62.4%. Intravenous drug use and homosexuality were identified as major transmission risk factors. GBV-C/HGV RNA prevalence was associated with hepatitis B coinfection, whereas antibodies to E2 were associated with older age and lower CD4+ cell counts. GBV-C/HGV infection was frequent in this group of HIV-infected patients and was associated with older age, lower CD4 + cell counts, and the presence of hepatitis B surface antigen.