Taiwan is an endemic area for hepatitis B virus (HBV) infection, which is responsible for up to 80% of chronic liver diseases there. In contrast to an HBV carrier rate of 15-20% in the general population, only 1% of its population are seropositive for anti-HCV. To evaluate the role of HCV infection in chronic liver diseases in Taiwan, serum anti-HCV was studied using an enzyme immunoassay in 123 "healthy" administration staff of the hospital, 724 hepatitis B surface antigen (HBsAg)-positive and 157 HBsAg-negative patients with chronic liver disease. The prevalence of anti-HCV was 0.8% in the hospital staff, 24.3% in HBsAg-positive and 80.9% in HBsAg-negative patients with chronic liver diseases. Anti-HCV was positive in 10 (9.6%) of 104 HBsAg-positive and 31 (77.5%) of 40 HBsAg-negative patients with inactive chronic hepatitis; 94 (27.2%) of 346 HBsAg-positive and 53 (85.5%) of 62 HBsAg-negative patients with active chronic hepatitis; 49 (26.1%) of 181 HBsAg-positive and 33 (86.8%) of 38 HBsAg-negative patients with cirrhosis; 23 (26.7%) pf 86 HBsAg-positive and 10 (58.8%) of 17 HBsAg-negative patients with hepatocellular carcinoma. In HCV infected HBsAg-positive patients, the optical density was usually lower, and anti-HCV became negative in 27% on follow-up. HCV infection tends to occur more frequently in older, HBeAg-negative and anti-HD-positive patients with chronic HBV infection. It is concluded that HCV not only is the major agent for non-B chronic liver diseases but also plays a significant role in HBsAg-positive chronic liver diseases in Taiwan.