Most studies on the association between antibodies against hepatitis C virus (anti-HCV) and primary liver cancer (PLC) were limited to case-series, or cross-sectional case-control studies leaving a controversy on causal temporality. A nested case-control study on 38 newly-developed PLC patients and 152 matched controls selected from a cohort of 9,775 men in Taiwan recruited from September, 1984, to February, 1986, was carried out to examine the relation between HCV infection and PLC. Case-control pairs were matched on age (±1 year), residence, and the date at recruitment. Serum samples collected from study subjects at the initial recruitment were examined for anti-HCV by enzyme immunoassay and hepatitis B surface antigen (HBsAg) by reverse passive hemagglutination assay combined with radioimmunoassay. History of cigarette smoking, alcohol consumption, vegetable consumption, vegetarian habit, and chronic liver diseases were also obtained through standardized interviews according to a structured questionnaire at the recruitment. After adjusting for HBsAg status and other risk factors, the anti-HCV was significantly associated with the development of PLC showing a multivariate-adjusted relative risk of 88.24. The results suggest that HCV infection may play an important role in the etiology of human PLC in Taiwan. © 1994 Wiley-Liss, Inc.