The role of quantitative viral load in development of hepatocellular carcinoma (HCC) among chronic hepatitis B virus (HBV) carriers was evaluated using real-time PCR (TaqMan PCR), a highly sensitive method for quantitative detection of HBV DNA. Serum samples collected at study entry from HCC cases and matched controls were chosen separately from ongoing prospective cohort studies in Senegal, West Africa, and Haimen City, China. For 14 HCC cases and 28 controls from Senegal, the relative risk (RR, 95% CI) of HCC was 15.6 (2.0–124.3) for those positive by the TaqMan PCR assay. Average length of follow-up (study entry to death from HCC) among cases was 2.8 (±1.6) years. The paired median difference between cases and controls was 3.8 × 104 virions/ml, with cases higher (P = 0.09). In order to clarify the relationship with lower-titer viremia, we selected 55 cases and 55 matched controls from the Chinese cohort all negative for serum HBV DNA by conventional dot blot hybridization. In this group, the RR associated with HBV DNA positivity by TaqMan PCR was 3.1 (1.1–9.2), with an average duration of follow-up of 3.3 (±2.1) years. The median difference in quantitative viremia between cases and controls was 6.0×104 virions/ml, with cases higher (P < 0.0001). Increased risk appeared to be confined to subjects with viral loads >2.3 × 104 virions/ml. In conclusion, HBV viremia, except perhaps at extremely low levels, is associated with increased risk for HCC in prospective studies of chronic carriers in two disparate populations. J. Med. Virol. 72:35–40, 2004. © 2004 Wiley-Liss, Inc.