To assess the safety, antiviral activity and clinical benefit of lamivudine (3-thiacytidine: 3TC) in chronic hepatitis B, we will treat 60 patients with oral lamivudine in a dose of 100 mg daily for up to five years. Lamivudine is a nucleoside analogue which is used extensively in patients with HIV infection and is being studied in controlled trials in chronic hepatitis B. In this study, we will evaluate lamivudine in patients with four different forms of chronic hepatitis B: (A) Atypical serology (HBeAg negative), (B) extra-hepatic manifestations, (C) chronic delta hepatitis, (D) typical HBeAg- positive chronic hepatitis B. After evaluation and liver biopsy, patients will receive lamivudine, 100 mg orally once daily for 1 year, being monitored at regular intervals for symptoms of liver disease, side effects of lamivudine, serum biochemical and hematologic indices, and serologic markers of hepatitis B (and D) virus replication. At one year, patients will have a repeat medical evaluation and liver biopsy. If there is virologic, biochemical or histologic evidence of benefit, therapy will be continued thereafter for up to 5 years. Patients who develop viral resistance to lamivudine may be offered therapy with higher doses of lamivudine (300 mg per day). The activity of lamivudine will be assessed by changes in levels of HBV DNA or HDV RNA during treatment and its efficacy by loss of viral markers and improvements in aminotransferases and liver histology.