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Adefovir dipivoxil was recently approved for the treatment of wild-type and lamivudine-resistant hepatitis B virus (HBV) infection. Tenofovir disoproxil fumarate, a congender of adefovir that is used in the treatment of HIV infected patients, has recently been shown to also be effective in patients with lamivudine-resistant HBV infection. We therefore compared the two substances in a study of 53 patients defined by high HBV DNA (>6 log10 copies/mL) levels and genotypic evidence of lamivudine resistance. Thirty-five patients received tenofovir for 72 to 130 weeks, and 18 received adefovir for 60 to 80 weeks. Changes in HBV DNA levels were followed for the complete period of 48 weeks. Early viral kinetics were compared on matched subgroups of 5 patients each. Individually, all tenofovir-treated patients showed a strong and early suppression of HBV DNA within a few weeks whether they were coinfected with HIV or were without comorbidity. In contrast, considerable individual variations in HBV DNA decline were observed in the adefovir group. Thus at week 48, only 44% of these patients had HBV DNA levels below 10(5) copies/mL in contrast to 100% of the tenofovir-treated patients (P = .001). No severe side effects were noticed in either group. No evidence of phenotypic viral resistance could be demonstrated in the tenofovir-treated patients in the long term (up to 130 weeks). In conclusion, tenofovir may become an effective alternative for the treatment of patients with lamivudine-resistant HBV infection.