We performed a case-control study to assess the association of hepatocellular carcinoma (HCC) with hepatitis B virus (HBV) and hepatitis C virus (HCV) infection and alcohol drinking. We recruited as cases 172 subjects with an initial diagnosis of HCC, who were admitted to the two major hospitals in the province of Brescia, northern Italy, and 332 subjects, sex-, age-, and hospital-matched, who were admitted to the Departments of Ophthalmology, Dermatology, Urology, Cardiology, and Internal Medicine, as controls. Of the HCC cases, 23.8% were positive for HBsAg and 37.8% for HCV RNA; among the controls, 5.4% were positive for HBsAg and 4.8% for HCV RNA. History of heavy alcohol intake (>80 g of ethanol per day for at least 5 years) was found among 58.1% of the cases and among 36.4% of the controls. The relative risks (RRs) for HBsAg, HCV RNA positivity, and heavy alcohol intake were, respectively: 11.4 (95% confidence interval: 5.7-22.8), 23.2 (95% confidence interval: 11.8-45.7), and 4.6 (95% confidence interval: 2.7-7.8). Positive interactions (synergisms) between both HBsAg positivity and HCV RNA positivity and heavy alcohol intake were found, suggesting more than additive effects of viral infections and alcohol drinking on the risk of HCC. Infection with HCV genotype 1b showed a higher risk than type 2 (RR = 2.9; 95% confidence interval: 0.9-10), suggesting a major role for the former type in causing HCC. On the basis of population attributable risks (AR), heavy alcohol intake seems to be the single most relevant cause of HCC in this area (AR: 45%), followed by HCV (AR: 36%), and HBV (AR: 22%) infection.