Primary cardiac lymphoma is defined as a non-Hodgkin's lymphoma mainly located in the heart and/or the pericardium. It is rare and affects elderly men. Common manifestations are pericardial effusion and heart failure. Diagnosis is usually late and prognosis is poor. We report a case of a patient with a large primary cardiac lymphoma who presented with chest pain and negative T-waves in electrocardiogram. Transthoracic echocardiogram showed a large mass in the right atrium and right ventricle while transesophageal echocardiogram also revealed the presence of large mobile masses in the right atrium, which were considered to be thrombi. Tissue biopsy showed a high-grade B-cell diffuse lymphocytic lymphoma. The patient was treated with chemotherapy and radiation with complete remission and prolonged survival.