Patients with angioimmunoblastic T-cell lymphoma can have profound immune dysfunction and immunodeficiency. Epstein-Barr virus-driven B-cell lymphoid proliferation can occur in angioimmunoblastic T-cell lymphoma, as in other immunodeficiency states. However, few cases of Epstein-Barr virus-positive B-cell lymphoma arising in patients with preexisting angioimmunoblastic T-cell lymphoma have been reported. We report a case of angioimmunoblastic T-cell lymphoma in which diffuse large B-cell lymphoma developed 56 months after the diagnosis of angioimmunoblastic T-cell lymphoma. The patient survived for 9 years after the initial diagnosis of angioimmunoblastic T-cell lymphoma, and molecular studies performed on multiple biopsy specimens during this period revealed the dynamic nature of clonal lymphoid expansion. Epstein-Barr virus latent membrane protein 1 and Epstein-Barr virus-encoded RNA were detected in the diffuse large B-cell lymphoma, suggesting that Epstein-Barr virus may have played a role in the pathogenesis of the diffuse large B-cell lymphoma.