We have investigated several groups of Thai patients diagnosed with chronic liver disease including chronic hepatitis, cirrhosis and hepatocellular carcinoma, as well as cholangiocarcinoma, for the prevalence of infection with either one of the hepatitis viruses B, C, G and the novel hepatitis virus TT (TTV). The 168 patients tested comprised 120 men and 48 women with their median age ranging from 42.3 to 62.3 years. Screening for antibodies to HBV and HCV was performed by a commercially available serological test kit, for the presence of HBV and TTV DNA by PCR, and of HCV and HGV RNA by RT-PCR, respectively. There was a clear two-fold higher prevalence of HBV (49%) over HCV (27%) infection and a four-fold higher frequency compared to HGV (13%) and TTV (11%) infection, respectively, in those individuals with chronic hepatitis, cirrhosis, and hepatocellular carcinoma, whereas all but one patient with cholangiocarcinoma the etiology of which has been ascribed to parasitic infestation, were free of all viral markers. In Thailand chronic HBV, and to a lesser extent, chronic HCV infection represent the two most common causes of hepatitis potentially proceeding to chronic liver disease, whereas the clinical significance pertinent to HGV and TTV remains to be elucidated.