Serum hepatitis B virus (HBV) DNA is now the most important and reliable marker for monitoring hepatitis B viral replication. Quantitative detection of HBV DNA in serum is based on commercial standardized molecular hybridization test systems. We compared two hybridization assays, the Digene Hybrid Capture assay (Digene Diagnostics, Beltsville, MD) and the Abbott HBV DNA assay (Abbott Laboratories, North Chicago, IL, USA) with the polymerase chain reaction (PCR) technique, for detection and quantitative measurement of serum HBV DNA. Forty-two patients with various HBV serological marker profiles were included in this study. The patients were divided into four groups according to their HBV DNA values after HBV DNA determination in the serum by the Abbott assay. For each patient HBV DNA was then determined by the Digene assay and by PCR. In the case of Digene and PCR there was a 97.6% correspondence in the outcome of the two methods, whereas in the Abbott assay and PCR there was only 69% correspondence. The McNemar test of symmetry showed no statistically significant difference between the Digene assay and PCR, whereas there was a significant difference (P < 0.01) in the Abbott assay and PCR. For low positive HBV DNA values between 1.5 and 20 pg ml-1 the Abbott assay yields inconclusive results. Differences observed between the two hybridization assays underline the need for standardization of HBV DNA quantitation.