By using reverse transcription and PCR for NS3 and 5'-untranslated regions (5'UTR) of the viral genome, prevalence of GB virus C/hepatitis G virus (GBV-C/HGV) infection in Chiang Mai, Thailand, was studied. High prevalence of GBV-C/HGV infection was observed among intravenous drug users (32%) and hemodialyzed patients (25%). The prevalence was also considerably high among patients with chronic liver disease, such as chronic hepatitis (9%), liver cirrhosis (12%) and hepatocellular carcinoma (10%). On the other hand, the prevalence among healthy blood donors (1%) was significantly lower than that of the above high-risk groups. GBV-C/HGV RNA positivity was significantly higher in individuals with antibodies against hepatitis C virus (24%) than in those without (5%). Phylogenetic analysis of the 5'UTR sequences classified Thai GBV-C/HGV isolates into three groups; (i) a group of isolates that are commonly found in the United States and Europe, (ii) a group of isolates that are commonly found in Asia, and (iii) a group of novel sequence variants.