Study Comparing the Safety of Switching From Lamivudine to Adefovir Dipivoxil Versus Overlapping Lamivudine and Adefovir Before Adefovir Dipivoxil Monotherapy in Patients With Chronic Hepatitis B

Inclusion Criteria: * Males and females ≥ 18 years of age with chronic hepatitis B * Hepatitis B surface antigen (HBsAg)(+) for a minimum of 6 months prior to entry * Hepatitis B envelope antigen (HBeAg)(+) or (-) at baseline * Patients having previously received LAM for at least 24 weeks * Patients with compensated liver function (Child-Pugh score ≤ 6) Exclusion Criteria: * Any serious or active medical or psychiatric illness which, in the opinion of the investigator, would interfere with patient treatment, assessment or compliance with the protocol. * Received immunoglobulins, interferon or other immune or cytokine-based therapies with possible activity in hepatitis B disease within 6 months prior to study screening. * Organ or bone marrow transplant recipients. * Evidence of active liver disease due to other causes (e.g., Wilson's disease, hemochromatosis, autoimmune hepatitis, hepatitis C or hepatitis D co-infection) * Patients taking parenteral (intravenous or intramuscular or subcutaneous) or oral steroids, immuno-suppressant therapies or chemotherapeutic agents within 2 months of study screening or expected to receive these agents during the course of the study. * Previous participation in an investigational trial involving administration of any investigational compound within 2 months prior to the study screening or those who received anti-HBV therapy other than lamivudine within the previous 3 months (e.g. anabolic steroids, ketaconazole, itraconazole, isoniazid, rifampin, rifabutin, simvastatin, lovastatin) * Clinically relevant alcohol or drug use or history of alcohol or drug use considered by the investigator to be sufficient to hinder compliance with treatment, follow up procedures or evaluation of adverse events * Lactating females or females with a positive serum pregnancy test. * Females of childbearing potential (post-puberty) unwilling or unable to have pregnancy testing at any study visit * Therapy with nephrotoxic drugs (e.g. aminoglycosides, amphotericin B, vancomycin, cidofovir, foscarnet, cisplatin pentamidine, tacrolimus, cyclosporine) or competitors of renal excretion (e.g. probenecid) within 2 months prior to study screening or the expectation that subject will receive these during the course of the study. * The use of antiviral therapy with agents demonstrating potential anti-HBV activity other than lamivudine within the previous 3 months (e.g. famciclovir, lobucavir, emtricitabine, DAPD, L-FMAU, entecavir, ganciclovir or others). * History of hypersensitivity to nucleoside and/or nucleotide analogues. * Clinical, ultrasonographic or radiologic evidence of hepatic mass suggestive of hepatocellular carcinoma. * Serum alphafetoprotein (AFP) \> 50 ng/mL at the first screening visit. However, if the AFP level is \> 50 ng/mL at the first screening visit, but has remained stable or decreased over the 6 months preceding the first screening visit, and if there is no radiologic or ultrasonic evidence of hepatic mass(es) suggestive of hepatocellular carcinoma, the patient will be allowed to enroll. * Inability to comply with study requirements.