Two research groups recently and independently, isolated a hepatotropic flavivirus from human sera. The two viruses, named GB virus C and hepatitis G virus (HGV), were subsequently discovered to represent the same virus, which was associated with acute and chronic hepatitis of the non-A-E type. The prevalences of infection with HGV have now been investigated in various groups of the Thai population, some of which [e.g. thalassaemic children, patients with chronic liver disease, carriers of antibodies to hepatitis B virus and/or hepatitis C virus (HCV), prostitutes and intravenous-drug users (IVDU)] were assumed to be at high risk. Samples of sera were investigated by reverse-transcriptase PCR, using four primers created from the 5' untranslated region of HGV. The prevalence of HGV infection among the healthy controls (1%-5%) was found to be much less than that among thalassaemic children (32.6%), asymptomatic carriers of anti-HCV (20.4%), IVDU (18.2%), aplastic anaemia patients (14.3%) and prostitutes (10%), although similar to that in patients with chronic liver disorders. These results confirm a parenteral route of transmission for HGV and emphasise the need for further research to determine the clinical significance of this virus.