A 42-year-old male suffered from AML (M2) and achieved remission with chemotherapy. After that, he was successfully treated with allogeneic bone marrow transplantation. About eight months later, jaundice and general malaise developed and diagnosis of acute hepatitis B type was made based on laboratory findings. After 3 months of a conservative therapy, he recovered from the disease. During the clinical course of the hepatitis, B lymphocytes were increased to about 70% of peripheral blood lymphocytes (PBLs) transiently, and furthermore CD5 positive B lymphocytes occupied 12% of the PBLs at that time. This B lymphocytosis disappeared gradually along with the improvement of the hepatitis. The remarkable increase of B lymphocytes in PBLs was considered to be an abnormal reaction induced by HB virus infection, when his immune system was in the recovering phase after bone marrow transplantation.