Recent studies on the natural history of hepatitis B virus (HBV) infection have provided evidence for a close relationship between the phase of active viral replication and the development of hepatic lesions. The presence of HBeAg has been associated with active viral replication and underlying active liver disease, whereas seroconversion from HBeAg to anti-HBe is usually accompanied by remission of the disease activity. With the availability of the technique of molecular hybridization to detect HBV DNA in serum, it has now been confirmed that HBV may continue to replicate after seroconversion from HBeAg to anti-HBe. In the present study, serum HBV DNA was measured in cases of chronic HBV infection and the results were correlated with HBeAg/anti-HBe status, histological diagnosis and HBV DNA polymerase, In addition, the changes of serum HBV DNA were monitored before, during and after Ara-AMP(adenine-arabinoside 5`-monophosphate) treatment. The results were summarized as follows: 1) Among 99 HBsAg positive cases, serum HBV DNA was detected in 61(61. 1%), In 51 of 68(75.9%) HBeAg positive cases and 10 of 24(47.1%) anti-HBe positive, serum HBV DNA was detected. In 6 negative cases for both HBeAg and anti-HBe .and 1 positive for both, serum HBV DNA was not detected. 2) Among 9 HBsAg negative cases, serum HBV DNA was detected in 1(11. 1%). In 5 positive cases for both anti-HBc and anti-HBs serum HBV DNA was not detected, but in 1 of 4(25.0%) positive for anti-HBc alone serum HBV DNA was -detected. 3) The serum HBV DNA was detected in 4 of .11(36. 4%) asymptomatic healthy carrier, 11 of 17 (64. 7%) chronic persistent hepatitis and 46 of 70 (65. 7%) chronic active hepatitis or liver cirrhosis. 4) The levels of serum HBV DNA were below 50 pg per ml in 50 of 61(81. 9%) HBV DNA positive cases. There was no significant correlation between the level of HBV DNA and histologic diagnosis. 5) DNA polymerase activity was tested in 105 sera from 49 cases and the results were compared to serum HBV DNA. Serum HBV DNA was detected in 65 of 76(85. 5%) DNA polymerase positive sera and 6 of 29(20. 7%) DNA polymerase negative sera. Meanwhile serum HBV DNA was not detected in 11 of 76(14.5%) DNA polymerase positive sera and 23 of 29(79.3%) DNA polymerase negative sera. 6) Serum HBV DNA, DNA palymerase and HBeAg were serially checked in 12 patients with HBeAg positive chronic active hepatitis who received 4 weeks Ara-AMP therapy. Before receiving Ara-AMP, 9 of 12 cases were positive for serum HBV DNA and DNA polymerase and these all patients showed a quantitative response in serum HBV DNA and DNA polymerase. But in only 1 of 12 cases, HBeAg was converted to negative during the course. In conclusion, serum HBV DNA was found in 75. 0%(51/68) of HBeAg positive and in 47.1%(10.24) of anti-HBe positive cases. Serum HBV DNA was detected more frequently in cases with chronic liver disease than in asymptomatic healthy carrier, but there was no significant difference between chronic persistent hepatitis and chronic active hepatitis. And serum HBV DNA appeared to be a useful marker in monitoring of the effect of antiviral treatment.