GB virus C/hepatitis G virus (GBV-C/HGV) is a recently discovered flavivirus of still unknown pathogenic relevance. We examined traumatologic outpatients to determine GBV-C/HGV viremia for further epidemiological studies, as blood donors hitherto used as controls represent healthy individuals without risk factors. Anti-GBV-C/HGV antibodies were detectable in 13.2% (95% confidence interval [CI] 9.3-18.2) and GBV-C/HGV RNA was detectable in 4.5% (95% CI 2.4-8.2) of the outpatients. In chronic non-A-E hepatitis patients GBV-C/HGV viremia was detectable at a significantly higher level of 16.1% (95% CI 6.1-34.5), while the prevalence of anti-GBV-C/HGV antibodies was 12.9% (95% CI 4.2-30.8). The rate of GBV-C/HGV viremia in patients with malignant diseases (different types of tumors, blood recipients were excluded) was 12.5% (95% CI 8.4-18.1), a significant elevation compared to traumatologic outpatients. The seroprevalence in the tumor group was 22.1% (95% CI 16.7-28.6), also significantly elevated. Thus, there are two messages. Firstly, testing for GBV-C/HGV may be a useful extension of the diagnostic procedure of viral hepatitis. Secondly, common risk factors or etiologic relations of GBV-C/HGV and extrahepatic malignancies should be discussed.