To explore the etiological agent(s) of HBsAg negative chronic active hepatitis, serum HBV DNA was tested in thirty-six of such cases with polymerase chain reaction. In total, viral DNA was detected in 24 cases (67%), including 11 of the 12 cases with positive anti-HBc alone, four of the nine cases with positive anti-HBs and nine of the 15 cases without any HBV markers. The results suggest that in cases of HBsAg-negative chronic active hepatitis, the etiology may still be hepatitis B virus, especially in cases with positive anti-HBc. Furthermore, the cause of activated histology might be due to viral replication.