BACKGROUND: Several studies have demonstrated that quantitation of hepatitis B virus (HBV) DNA in sera of HBsAg-positive patients is more useful test for the assessment of infectivity and for the evaluation of disease status than previously utilized numerous serological markers and qualitative polymerase chain reaction for the detection of HBV DNA. We tried to measure serum HBV DNA using a branched DNA (bDNA) signal amplification assay, which is recently introduced and known to be a simple and nonradioisotopic method. METHODS: Total forty patients with HBsAg were randomly selected and serum HBV DNA was measured with duplication using bDNA signal amplification assay (QUANTIPLEXTM HBV DNA ASSAY, Chiron, USA). Quantitation was determined from a standard curve and expressed as HBV DNA equivalents/mL (Eq/mL; 1 Eq = 1 molecule of the primary HBV DNA standard). Serum HBeAg, aspartate aminotransferase (AST), alanine aminotransferase (ALT) , and soluble interleukin-2 receptor (sIL-2R) were compared with HBV DNA. RESULTS: Serum HBV DNA was quantitated in 13 patients (32.5%) (range 6.4x106-7.4x109 Eq/mL, mean 1.8x109 Eq/mL, CV 8.1%). All eleven patients (100%) with both HBsAg and HBeAg an4 2 of 29 patients (6.9%) with HBsAg but not with HBeAg showed measurable HBV DNA (p < 0.001). In addition, serum levels of AST, ALT, and sIL-2R were significantly higher in HBV DNA measured patients compared with those of unmeasured patients. CONCLUSIONS: Above results show that more than half the HBsAg-positive patients do not have enough HBV DNA which is measurable with boNA signal amplification assay but all of HBeAg-positive patients and some of HBeAg-negative patients do. In addition, HBV DNA quantitation might be correlated with the disease activity in HBsAg-positive patients because serum levels of AST, ALT, and sIL-2R are higher in patients measured with HBV DNA than unmeasured.