Inclusion Criteria:

1. =/> 16 years of age (or minimum age required in a given country).
2. Prior lamivudine therapy for a cumulative period of > 24 weeks or genotypic evidence of lamivudine resistance.
3. Clinical evidence of lamivudine-resistant hepatitis B defined as positive serum hepatitis B virus (HBV) DNA greater than or equal to 10^6 copies/mL (PCR assay) and ALT greater than or equal to 1.2 X upper limit of normal (ULN) within 4 weeks of screening despite ongoing therapy with lamivudine.
4. Treating physician feels that the patient is at risk for disease progression.

5. Screening laboratory values measured as follows, within 28 days prior to the baseline visit:

   • Adequate hematologic function.
   • Absolute neutrophil count =/> 750/mm3, platelets =/> 50,000/mm3, hemoglobin =/> 7.5 g/dL.
6. Females of childbearing potential must have had a negative serum or urine pregnancy test during the screening period. Females must use effective method(s) of contraception during heterosexual intercourse while on adefovir dipivoxil and at least 30 days following treatment discontinuation.
7. Able to understand and sign the informed consent prior to undergoing study procedures and able to comply with the requirements of the study.
8. Patients co-infected with HIV, hepatitis C virus (HCV), or other viral infections will be eligible to participate provided they meet all other entry criteria.

Patients who do not meet these entry criteria but for whom the treating physician believes that chronic hepatitis B disease progression or premature death is likely to occur in the absence of early access to adefovir dipivoxil will be considered on a case-by-case basis by the Parexel medical monitor.

Exclusion Criteria:

1. Patients with any serious or active medical or psychiatric illness that, in the opinion of the investigator, would interfere with patient treatment, assessment, or compliance with the protocol or dosing requirements.
2. Patients with hypersensitivity to any of the components of the drug product.
3. Currently receiving nephrotoxic drugs (with the exception of cyclosporine or tacrolimus in patients post liver transplantation) such as aminoglycosides (e.g., amikacin, gentamicin, kanamycin, neomycin, netilmicin, streptomycin, tobramycin), conventional amphotericin B, intravenous (IV) vancomycin, cidofovir, IV foscarnet, cisplatin, or IV pentamidine OR competitors of renal excretion such as probenecid and sulfinpyrazone. These agents must be discontinued at least 7 days prior to starting treatment with adefovir dipivoxil.
4. Currently enrolled in another clinical trial of adefovir dipivoxil.
5. HIV and HBV co-infected patients receiving tenofovir disoproxil fumarate [Viread(R)] for their HIV disease.
6. Pregnant or lactating females.