The Efficacy of Switching to Entecavir (ETV) Monotherapy in Japanese Lamivudine (LVD)-Experienced Patients. Background: This study aims to determine the efficacy of switching to 0.5mg ETV daily in chronic hepatitis B (CHB) patients previously treated with LVD. Method: Retrospective analysis of CHB patients (n=134) previously on 100mg LVD daily and switched to 0.5mg ETV daily. Results: LVD-experienced patients were divided into three groups based on HBV viral load at time of switching to ETV (<2.6 log10 copies/mL; 2.6-5.0 log10 copies/mL and >5.0 log10 copies/mL). Detection of LVD-resistant virus at the time of switching was higher in the group with HBV DNA 2.6 log10 copies/mL (76% in both 2.6-5.0 and >5.0 log10 copies/mL groups versus 23% in <2.6log10 copies/mL group) and was higher in patients treated with LVD for 3 years (52% versus 24% for patients on <1 year of LVD). A year after switching to 0.5mg ETV daily, HBV DNA undetectable rates were 100% (42/42), 94% (17/18) and 43% (6/14) for <2.6, 2.6-5.0 and >5.0 log10 copies/mL groups, respectively. ALT normalization occurred in more than 90% patients at the end of the first year of switching to ETV for all three patient groups. Only one patient in the 2.6-5.0 log10 copies/mL group, who had LVD-resistant mutants at the time of switching, developed ETV resistance during follow-up. Conclusion: Switching from LVD to ETV maintains or improves viral suppression and ALT normalization, especially in patients with viral load <5.0 log10 copies/mL.