This study evaluated the efficacy of adefovir (ADV) plus lamivudine (LAM) or ADV add-on therapy for patients with entecavir (ETV)-refractory hepatitis B infection. Twenty-nine ETV-resistant and 8 patients with suboptimal response to ETV were enrolled. Twenty-seven patients received ADV + LAM therapy and 10 patients received ADV + ETV therapy for >24 weeks. In 29 patients who were ETV-resistant, the mean reduction in HBV DNA levels at 24 weeks was not different between the ADV + LAM and ADV + ETV groups (−1.98 log10 IU/ml vs. −2.16 log10 IU/ml; P = 0.792). Primary non-response was observed in 52.2% (12/23) of ADV + LAM group and 33.3% (2/6) of ADV + ETV group (P = 0.651). Initial virologic response (IVR) was observed in 17.4% (4/23) of ADV + LAM group and 33.3% (2/6) of ETV + ADV group (P = 0.362). In eight patients with suboptimal response to ETV, the ADV + ETV group had a greater reduction in HBV DNA at 24 and 48 weeks than the ADV + LAM group (−2.29 log10 IU/ml vs. −0.09 log10 IU/ml and −2.04 log10 IU/ml vs. −0.72 log10 IU/ml; P = 0.020 and P = 0.012, respectively). Primary non-response and IVR did not significantly differ between the two groups [100% (4/4) vs. 50% (2/4) and 0% (0/4) vs. 50% (2/4); P = 0.429 and P = 0.429, respectively]. The antiviral efficacies of ADV-based therapy with ETV or LAM for patients with ETV-resistant hepatitis B were limited and did not differ between the two groups. However, adding ADV to ETV may be more effective than ADV + LAM therapy in patients with suboptimal virologic response to ETV. J. Med. Virol. 84:18–25, 2011. © 2011 Wiley Periodicals, Inc.