Cardiac tamponade due to lymphomatous involvement of the heart is a dramatic and unusual complication. Because of their nonspecific clinical presentation, these tumors are seldom diagnosed antemortem. We report the case of a patient with AIDS who presented with signs and symptoms of cardiac tamponade. Emergency pericardiocentesis followed by staging studies revealed large cell B-lymphocyte lymphoma confined to the pericardial space. With combination chemotherapy, a durable complete response was obtained. This case illustrates the potential benefit of aggressive treatment of extranodal non-Hodgkin's lymphoma in a patient with AIDS. The case is of particular interest because of the unusual development of isolated pericardial involvement as the sentinel sign of lymphoma.