Cardiac involvement, as an initial presentation of malignant lymphoma is rare. We report a 42-year-old Taiwanese man with frequent syncope. Routine electrocardiography disclosed complete atrioventricular block. Before permanent pacemaker implantation was undertaken, transthoracic echocardiography revealed a solid mass infiltrating the left atrium and interatrial septum. Transesophageal echocardiography proved cardiac neoplasm with biatrial infiltration. Endomyocardial tissue biopsy was performed under angiography guide. Pathology diagnosis unraveled large B-cell lymphoma. Despite aggressive chemotherapy, the patient's condition worsened and he died 2 months later.