Acute aggravation of chronic hepatitis B is life-threatening, but there has been no acceptable treatment modality except for liver transplantation. Here, we report the effectiveness and safety of lamivudine in treating chronic hepatitis B of acute accelerating course. Three patients with chronic hepatitis B and one patient with liver cirrhosis due to hepatitis B were enrolled. All the cases showed hyperbilirubinemia and was in acute accelerating course. After the beginning of lamivudine treatment, HBV DNA in serum became undetectable in 2-6 weeks, and serum levels of aspartate aminotransferase and alanine aminotransferase became normal in 3 months in all cases. In two of them, HBeAg became negative at 7 weeks and 6 months after therapy, respectively. Lamivudine treatment in these patients was generally well tolerated. Lamivudine was safe and effective in treating chronic hepatitis B of accelerating course. (Kor J Gastroenterol 2000;35:806 - 814)