This study will evaluate the long-term safety and effectiveness of lamivudine therapy and the possibility of stopping therapy in patients whose hepatitis B is chronic, that is, long lasting, and which has responded to treatment. Chronic hepatitis B, caused by a virus, is a common form of liver disease affecting about 1 million Americans and about 5 percent of the world's population. Health effects include a continuous state of being infectious and the risk of transmitting hepatitis to other people, symptoms of liver disease, and development of cirrhosis-that is, severe damage to the liver-and liver cancer. Lamivudine is a medication that blocks hepatitis B effectively but does not make it disappear completely. Scientists believe that the immune system must also be active to rid the body of the last traces of hepatitis B.