In this paper we describe a cohort of patients treated with lamivudine, a reverse transcriptase inhibitor with a strong virus-suppressive effect on the hepatitis B virus. Eighty-nine patients were included in the intention-to-treat analysis. Evaluation with the Kaplan-Meier method was based on response to therapy of HBV DNA levels as well as normalization of transaminases. Subgroup evaluation based on the per protocol data was performed on two groups within this cohort based on the HBV DNA level at the end of therapy. During a 52-week treatment period, the chance of being HBV DNA negative at one point in time, as measured by the Digene Hybrid Capture assay (HCS) (limit of detection 1.5 x 10(6) geq/ml), the quantitative PCR assay (Q PCR) (limit of detection 1000 geq/ml), or the chance of normalization of transaminases at one point in time is 78%, 57% and 66%, respectively. Nineteen out of 73 patients who had continuing active viral replication after at least 24 weeks of lamivudine therapy were evaluated for the emergence of mutations resistant to lamivudine. In 3 out of 19 patients a mutation in the highly conservative YMDD region of the polymerase gene was detected. Baseline viral load in this group was significantly higher compared to the other 54 patients who were treated for 24 weeks or longer. Thirty-one out of 73 patients (46%) became negative by Q PCR. HBV DNA level at start of treatment was significantly lower compared to the 42 patients who remained HBV DNA positive. Eleven consecutive patients within this group who became negative by qualitative PCR (limit of detection 400 geq/ml) were evaluated to obtain characteristics for lamivudine withdrawal. Ten out of 11 patients became HBeAg negative with anti-HBe in 6. HBeAg in the liver biopsy was negative in 10 out of 11 patients; 9 out of 10 obtained biopsies were positive for HBV DNA, indicating low-level viral replication. In two patients in whom lamivudine was withdrawn, rebound of virus occurred. Further research is needed to obtain better insight into the variability in response. This might lead to a tailor-made individualized treatment regimen.