A 24-year-old Japanese male was admitted to our hospital because of lymphadenopathy in his left neck. He had a nine-year history of chronic renal failure, and had received an ABO-mismatched renal allograft and splenectomy in August 2000 after one year of hemodialysis treatment. After renal transplantation, he was treated with FK506, methylprednisolone (mPSL), and mycophenolate mofetil (MMF) as an immunosuppressants for his graft maintenance. On admission, April 2001, he underwent lymphadenectomy, and the immunohistochemical studies revealed that the tumor cells expressed EBV-LMP and EBNA-2 antigens with the histology of diffuse large B-cell lymphoma. Our diagnosis was an Epstein-Barr virus (EBV)-associated posttransplant lymphoproliferative disease (PTLD), and we reduced the dose of immunosuppressive agents and treated the patient with rituximab. In this case, there may have been two principal risk factors associated with PTLD: first, the patient was treated with higher levels of immunosuppressive agents because of the ABO-mismatched transplantation, and second, he was an EBV-seronegative recipient at the time of pretransplantation.