We report a case of secondary diffuse large B-cell lymphoma (DLBCL) after azacitidine (AZA) treatment in a 63-years-old man with myelodysplastic syndrome. The patient suffered from febrile neutropenia after 10 cycles of AZA treatment. Despite the performance of a whole-body CT scan, which showed a multifocal low-density area in the liver and a multifocal nodular shadow in the lung, no malignant neoplasms could be detected. An autopsy was performed 6 months later, and a histopathological examination of the lesions of the liver and lung revealed the infiltration of large round-shaped tumor cells with necrotizing lesions. Immunohistochemically, the tumor cells were positive for CD20 and EBER, indicating EBV-positive DLBCL as a secondary malignancy.