Primary lymphoma of the heart is an uncommon malignancy usually recognized at autopsy or fatal within a few weeks of diagnosis. Recently, it was reported in patients with acquired immune deficiency syndrome. A patient with diffuse large cell lymphoma of the heart is reported who had chest pain and rapidly evolving cardiac arrhythmias. The human immune deficiency virus antibody test was negative. Because of an aggressive diagnostic approach, therapy with cyclophosphamide, doxorubicin, vincristine, and prednisone was started on the third day after diagnosis. The patient has survived 18 months with an objective response. To the authors' knowledge, this is the longest reported survival in primary cardiac lymphoma. The diagnosis in this patient was aided by excellent tumor delineation by nuclear magnetic resonance scanning. The authors believe that better survival in this patient was a result of prompt diagnosis and treatment because the behavior of the lymphoma was similar to aggressive lymphomas arising elsewhere. Cancer 68:2289–2292, 1991.