The biopsy findings of malignant lymphoma involving the pericardium in a 69-year-old male were reported. The initial presenting symptoms were heart failure followed by rapidly progressive complete heart block. Pacemaker was inserted. Chest roentgenogram showed a marked left pleural effusion. An echocardiogram demonstrated pericardial fluid. The diagnosis of lymphoma was suggested by gallium isotope study with markedly increased isotope uptake in heart, and was subsequently confirmed by pericardial biopsy. After chemotherapy the pleural effusion was decreased and the uptake of gallium disappeared. While the diagnosis of cardiac involved lymphoma was hard antemortem, experience in this case suggests that gallium isotope study and pericardial biopsy are helpful in establishing the correct diagnosis.