The prevalences of serological markers of hepatitis B virus (HBV) and antibody to hepatitis C virus (anti-HCV) were determined in 168 patients (135 males and 33 females), aged 19-79 years (mean = 50.8) in Thailand. Of these, 33 had chronic persistent hepatitis, 35 chronic active hepatitis, 50 cirrhosis and 50 hepatocellular carcinoma (HCC). Seromarkers for either HBV or anti-HCV or both were detected in 140 (83.3%), 3 (1.8%) and 18 (10.7%) patients, respectively, but 7 (4.2%) were sero-negative for both viruses. The overall prevalence of anti-HCV was 12.5% but was significantly lower in HCC (2%) compared to the other 3 groups of liver disease (12-21.5%, p less than or equal to 0.05) and in HBsAg positive (5%) compared to HBsAg negative (30%) patients (p less than 0.001). After 0.5-9 years follow-up of all anti-HCV positive patients, 2 died and another 6 had progressive liver disease. The prevalence of coexistent HBV seromarkers was similar in patients with a progressive (87.5%) and a stable clinical course (92.3%) (p = 0.62). A higher proportion of the anti-HCV-positive patients with a progressive course had a history of blood transfusion [75.0% vs 46.1% (p = 0.20)]. These findings suggest that HBV is the most important etiologic virus associated with chronic liver disease and HCC in Thailand, but HCV may play a role particularly in HBsAg-negative patients.