Patients ages 18 and older who have chronic hepatitis B and are being treated with lamivudine may be eligible to participate in this study. They will undergo a medical history and physical examination and will be given lamivudine in 100 mg tablets to be taken as one tablet, once each day. Patients will be asked to return to the outpatient clinic every 3 months, when they will undergo a brief interview and measurement of vital signs-such as blood pressure, pulse, and body weight. During the visits, they will fill out questionnaires about any symptoms or side effects they have, and they will be seen by a doctor and have a brief medical history and examination. There will be a collection of blood for complete blood counts, liver enzymes, and hepatitis B virus. Extra blood tests may be done to analyze patients' immune reactions to hepatitis B. Patients will also receive refills of their lamivudine tablets. They will continue to be treated with lamivudine as long as it seems to control the hepatitis infection and liver disease.