Anti-HBc is found in virtually all patients who have been or are currently infected with HBV; therefore, it is considered as the most sensitive serological marker of HBV infection. However, presence of HBsAg without anti-HBc has sometimes been detected in blood donors, the clinical and virological significance of which has not been determined yet. This study was designed to investigate the presence of HBV replication by detecting HBV DNA using the dot blot hybridization and polymerase chain reaction (PCR) in the HBsAg-positive and anti-HBc-negative sera of patients with chronic liver disease. HBV DNA was detected in 62.5%, 75.0%, 87.5% of patients by dot blot analysis, PCR-ethidium bromide staining, PCR-Southern blot analysis, respectively. In conclusion, the high detection rate of HBV DNA in these sera highly suggested that active viral replication was going on in most of these patients, and it is necessary to define the presence of mutation in the precore and/or core region of HBV in these patients.