BACKGROUND The objective of this cross-sectional, nonrandomized, prospective study was to generate data on the prevalence of GB virus C (GBV-C)/hepatitis G virus (HGV) in a cohort of HIV-infected homosexuals from Munich. METHODS A total of 71 HIV-infected homosexual men were analyzed for prevalence of GBV-C RNA and antibodies to the E2 envelope glycoprotein (E2Ab). 475 healthy volunteer blood donors in southern Bavaria served as a control group. RESULTS The prevalence of GBV-C RNA was 27% (control group: 2.3%) and the prevalence of E2Ab was 35% (control group: 6%). The total prevalence for present and past infection was 62%. The differences between the HIV-infected patients and the control group were significant (p < 0.0001). GBV-C RNA and E2Ab were not detected simultaneously in any serum sample. The E2Ab positive patients were older than the GBV-C RNA positives (mean 46 years versus 39 years, p = 0.0350). The GBV-C RNA and E2Ab negative patients were older than the GBV-C RNA positives (mean 47 years versus 39 years, p = 0.0236). The E2Ab positive patients had suffered sexually transmitted diseases more frequently than the patients negative for markers of GBV-C infection (p = 0.0308). E2Ab positive patients also had higher mean levels of alanine aminotransferase compared to patients without evidence of GBV-C infection (p = 0.0164). 59.4% of all individuals were anti-HBc IgG positive. CONCLUSIONS The data can be interpreted as indirect evidence for sexual transmission of GBV-C.