Chronicinfection with the hepatitis B virus (HBV) is the most important risk factor for hepatocellular carcinoma (HCC). However, determinants of HCC risk in infected individuals are not well understood. We prospectively evaluated the association between acquired HBV 1762 T/1764 A double mutations and HCC risk among 49 incident HCC cases and 97 controls with seropositive hepatitis B surface antigen at baseline from a cohort of 18,244 men in Shanghai, China, enrolled during 1986 to 1989. Compared with HBV carriers without the mutations, chronic HBV carriers with the HBV 1762 T/1764 A double mutations experienced an elevated risk of HCC (odds ratio, 2.47; 95% confidence interval, 1.04-5.85; P = 0.04). Risk increased with increasing copies of the double mutations; men with ≥500 copies/μL serum had an odds ratio of 14.57 (95% confidence interval, 2.41-87.98) relative to those without the double mutations (P trend = 0.004). Thus, the HBV 1762 T/1764 A double mutation is a codeterminant of HCC risk for people chronically infected with HBV.