A case of primary malignant lymphoma of the heart diagnosed cytologically is reported. The patient presented with pericardial tamponade and ventricular arrhythmias and developed rapidly progressive and intractable cardiac failure. Two-dimensional echo-cardiography, which demonstrated the pericardial effusion, showed progressive impairment of the left ventricular contraction. The pericardial fluid contained malignant lymphoid cells. Despite vigorous treatment and chemotherapy, the patient died within 15 days; postmortem examination showed malignant lymphoma confined to the myocardium.