We observed a 45-year-old man with acute hepatitis B while receiving treatment of chronic graft-versus-host disease (GVHD) of the liver. When he developed a sudden elevation of serum aminotransferases 17 months after bone marrow transplantation, he was under immunosuppressive therapy consisting of cyclosporin A against chronic GVHD of the liver. Serologic tests for hepatitis B virus (HBV) showed no reactivation but de novo acute infection. The serum levels of aminotransferases after elevation of biliary tract enzymes increased mildly. However, icterus was not observed in his sequential course. A liver biopsy specimen revealed mild acute liver injury accompanied by slight degeneration of bile ducts. It is presumed that owing to immunosuppressive therapy, his liver dysfunction was relatively mild, and the hepatitis became quiescent without becoming serious. On the other hand, the serum of the patient remained hepatitis B surface antigen positive for more than 1 year after the onset of the hepatitis, which suggested chronicity of HBV infection.