Lamivudine has recently been registered for the treatment of chronic hepatitis B patients. The main therapeutic outcome in the studies on which the registration was based was a drop of HBV DNA below 107 genome equivalents/ml, the level of detection of the insensitive Abbott Genostics assay. However, as we have reported previously, with the use of sensitive PCR-based assays, individual differences in virological response to lamivudine can be detected. As a first step in analysing the chain of events after oral intake of lamivudine we modified and validated a highpressure liquid chromatography (HPLC) method to evaluate lamivudine plasma levels. Lamivudine levels in chronic hepatitis B patients who participated in a study on the efficacy of lamivudine were comparable to our reference curve, which was derived from eight chronic hepatitis B patients. From the reference curve, a mean area under the curve (AUC) of 4994 mcg/l.h (SD 1524), a mean tmax of 42 minutes (SD 11), and a mean Cmax of 1.9 mg/l (SD0.70) were calculated. Lamivudine exerts its action as the active triphosphate inside the hepatocyte after extensive handling. Therefore, additional steps in the pharmacokinetic process should be evaluated to explore the potential mechanisms that are responsible for the diversity in quantitative HBV DNA response to lamivudine.