The identification of serum HBV DNA is very important for the assessment of the disease activity in persistent infection, for the evaluation of the infectivity of an individuals blood. The dot blot, however, has limited sensitivity and sometimes inconsistent with other serological markers and clinical settings. Using the most important recent advance in molecular biology, the polymerase chain reaction(PCR), specific DNA sequences can be amplified more than a million-fold in a few hours and with this technique the detection of the extreme low level of DNA is possible. This study was to determine sensitivity of the PCR for the detection of serum HBV DNA in comparison with dot blot analysis and to investigate the serum HBV DNA status and clinical significance of PCR in patients with chronic HBsAg positive liver disease. The subjects of this study were 17 patients with asymptomatic HBsAg carriers(9 HBeAg positive patients, 8 anti-HBe positive patients), 91 chronic hepatitis B(50 HBeAg positive patients, 41 anti-HBe positive patients), 57 liver cirrhosis(21 HBeAg positive patients, 36 anti-HBe positive patients), 27 hepatocellular carcinoma(10 HBeAg positive patients, 17 anti-HBe positive patients). The results were summerized as following; The detection rates of HBV DNA by dot blot, PCR were 58.9%, 72.2% in HBeAg positive patients, 34.3%, 53.9% in anti-HBe positive patients. The detection rates of HBV DNA by PCR in HBeAg negative patients were 25.0% in asymptomatic HBsAg carriers, 61.0% in chronic hepatitis B, 52.8% in liver cirrhosis, 52.9% in hepatocellular carcinoma. The positive rate for HBV DNA is a significant difference between HBeAg positive and negative asymptomatic HBsAg carriers, but not significantly difference in other groups. In conclusions, this study confirmed that the PCR is much more sensitive than the dot blot analysis in detecting the HBV DNA in the sera of patients with chronic liver disease. The presence of HBV DNA in the serum was detected by PCR with higher sensitivity and it suggested that active viral replication is still going on in most patients with chronic HBsAg positive liver disease irrespective of HBeAg/anti-HBe status, and PCR may be used as a prognostic factor in asymptomatic HBsAg carriers.