A 50-year-old woman received a diagnosis of diffuse large B-cell non-Hodgkin's lymphoma in 2000 and achieved complete remission. In April 2004, echocardiography and computed tomography examinations identified a tumor attached to the tricuspid valve and protruding within the right atrium. Bone marrow and lymph node biopsies showed a relapse of large cell lymphoma. The patient had a markedly elevated level of B-type natriuretic peptide (BNP) but a normal level of cardiac troponin I. The follow-up evaluation of the BNP level after chemotherapy showed that it had returned to within normal limits, and an echocardiogram showed regression of the tumor. Use of the BNP level as a monitor in the treatment of cardiac lymphoma has never been reported. This article is the first to report the use of BNP monitoring before and after chemotherapy to evaluate a patient with an unusual relapsed lymphoma with cardiac involvement.