Hepatitis B virus (HBV) infection remains to be one of the most prevailing infection in the world, causing chronic liver diseases. Although lamivudine has been effective to suppress HBV replication, longer durations of administration can lead to the emergence of drug-resistant mutant viruses, followed by reactivation of hepatic inflammation (breakthrough hepatitis). Moreover, the optimal period of administration as well as the effects of anti-viral nucleot(s)ide such as lamivudine, adefovir, and entecavir, has not been established. To evaluate the efficacy of the anti-viral effects of entecavir for lamivudine-resistant HBV, we administered entecavir sequentially in four patients with chronic HBV infection, who demonstrated the emergence of lamivudine-resistant HBV and histological active hepatitis. The antiviral effects were evaluated by the serum viral loads and biochemical laboratory data. After follow-up periods of more than 36 months, we found high incidence in the emergence of entecavir resistant mutants (3/4, i.e., 75%). An additional mutation at the 184th amino acid, different from the previously reported lamivudine-resistant mutations (80th, 180th, and 204th), seemed to have a close relationship with the induction of entecavir-resistant mutants at least for Japanese HBV genotype C. Our observation draws attention to the possibility that the usage of entecavir for lamivudine-resistant HBV could promptly induce entecavir-resistant mutations in addition to lamivudine-resistance.