A 53-year-old man presented with fulminant hepatitis due to de novo hepatitis B. He had been diagnosed previously with adult T-cell leukemia (ATL) and previously resolved hepatitis B virus infection. The ATL had been treated with cord blood transplantation (CBT). He developed fulminant hepatitis 18 months after CBT, 15 months after the withdrawal of immunosuppressants, and 10 months after vitreous injections of methotrexate for ATL-related retinal infiltration. The aggressive medical protocol included entecavir, prednisolone, plasma exchange, hemodialysis, and bilirubin adsorption. We herein report successful medical treatment for fulminant de novo hepatitis B in a patient considered unsuitable for liver transplantation.