We present a case of non-Hodgkin's lymphoma with massive cardiac involvement in a previously well 65-year-old man, presenting with pericardial tamponade and heart failure of recent onset. Results of echocardiography and of pericardial and pleural fluid cytology suggested the diagnosis. Within two weeks the patient's condition progressed to complete heart block and he died. Primary cardiac tumors are rare when compared with metastatic involvement of the heart. Their presentation includes congestive heart failure, cardiomegaly, pericardial effusion, and sudden death. The clinical diagnosis has seldom been made.