Although 25 to 36% of systemic lymphoma patients develop cardiac involvement, a primary lymphoma involving only the heart or pericardium is much less common. We detected an intracavitary mass in the right atrium and right ventricle in a 73-year-old man with dyspnea on exertion and generalized edema using transthoracic and transesophageal echocardiography. A thoracotomy was performed due to a possible cardiac tamponade, and a myocardial biopsy showed a malignant non-Hodgkin's lymphoma of the diffuse large B cell type. We report a rare case of a primary cardiac lymphoma detected using a transthoracic and transesophageal echocardiography in patient presenting with a massive pericardial effusion.