Viral load has emerged recently as a reliable marker of disease progression and therapeutic efficacy in chronic infections, including AIDS and hepatitis C. The clinical management of type B hepatitis could also be improved by monitoring viremia levels in patients with chronic liver disease undergoing anti-viral treatment. To address this question we evaluated the performance of a newly developed, quantitative PCR assay (Amplicor HBV Monitor test, Roche Diagnostic Systems) in the assessment of viremia changes over time in a group of 45 patients with chronic active hepatitis (CAH) who received interferon treatment. Of the 45 patients, 14 were HBsAg and anti-HBeAg positive and 31 HBsAg, HBeAg positive. Follow-up extended up to 24 months. An average of ten samples per patient were analyzed for levels of ALT, IgM anti-HBc (Abbott Laboratories), HBV DNA by in-house dot-blot hybridization and hybridization-capture assays (HBV-DNA hybrid capture kit, Murex Diagnostics) and by Amplicor HBV Monitor. A sustained biochemical response was observed at the end of treatment in 12 HBeAg-positive and in seven anti-HBeAg positive patients. This was accompanied by the disappearance of HBeAg and of HBV DNA (hybridization assays) in all cases and by the clearance of IgM anti-HBc in 70% of the cases. Viremia (quantitative PCR assay) became undetectable only in 25-30% of cases and was associated with the loss of HBsAg. A good correlation was observed between the time course of IgM anti-HBc, quantitative PCR and dot-blot hybridization although the latter missed 33% of viremic samples. Together, these results indicate that the Amplicor HBV Monitor test is a robust and standardized assay for quantifying HBV viremia levels in the range from 10(2) to 10(7) copies/ml. Compared to other current markers, viral load may provide additional clinical information by predicting long term virologic response and HBsAg clearance in patients with normal ALT at the end of interferon therapy.