We describe a case of primary cardiac lymphoma diagnosed by transvenous biopsy under fluoroscopic and transthoracic echocardiographic guidance. A 38-year-old man was admitted because of exertional dyspnea. ECG revealed complete atrioventricular block. Transthoracic echocardiography revealed a large mass attached to the interventricular septum and protruding into the right atrium. The patient underwent a right heart catheterization and a biopsy was taken from the mass using fluoroscopic and transthoracic echocardiographic guidance. Diagnosis of malignant lymphoma was established by the biopsy specimen. The use of transthoracic echo in conjunction with fluoroscopy may be useful for the diagnosis of intracardiac mass transvenously.