A case of the sick sinus syndrome and constrictive pericarditis with effusion (seroconstrictive pericarditis) as manifestations of lymphomatous involvement of the heart is presented. Autopsy findings documented infiltration of the sinus node by undifferentiated malignant lymphoma cells. The clinical course is described and problems with diagnosis and management of such a complication are discussed. This is one of the few cases in the literature in which an uncommon cause of conduction system disease and pericardial constriction was substantiated by electrocardiogram, angiography, and autopsy.