The newly cloned and characterized hepatitis GB virus-C (HGBV-C), which is the same virus as the independently discovered hepatitis G virus, has a global distribution, is transmitted parenterally, and causes chronic viremia. The pathological consequences of infection with HGBV-C are uncertain, and its hepatocarcinogenic potential is unknown. We used a case-control format to compare the prevalence of HGBV-C infection in 167 southern African blacks with hepatocellular carcinoma (HCC) and 167 race-, age-, and sex-matched hospital-based control subjects, and to test for possible interactive effects between this virus and hepatitis B and C viruses in the development of the tumor. The presence of HGBV-C ribonucleic acid was detected in serum samples by reverse transcription, amplification of the resulting complementary deoxyribonucleic acid by the polymerase chain reaction (PCR), and Southern hybridization using a probe from the NS3/helicase region of the genome. Serum samples were also tested for the presence of hepatitis B virus surface antigen, antibodies to hepatitis C virus, and hepatitis C virus ribonucleic acid. Individuals infected with HGBV-C did not have an increased relative risk of developing HCC (relative risk 0.9; 95% confidence limits 0.5, 1.7). Moreover, co-infection with HGBV-C did not further increase the risk of tumor development in patients who were chronically infected with hepatitis B and/or C viruses. HGBV-C is unrelated to hepatocellular carcinoma development in black Africans.