Primary cardiac lymphoma is very rare and clinical symptoms of cardiac involvement are unusual. The development of conduction defects as the first symptom of cardiac involvement is very uncommon. We report the case of a 57-year-old woman with syncope and complete atrioventricular block due to large B-cell primary cardiac lymphoma. The patient showed a refractory pericardial effusion. Transthoracic echocardiography revealed the presence of a mass in the pericardial space. The diagnosis of diffuse large B-cell lymphoma was made following open-chest biopsy of the heart. The clinical presentation of and the diagnostic approach to primary cardiac lymphoma are discussed. Atrioventricular block and refractory unexplained pericardial effusion and/or the existence of a cardiac mass should arouse the clinical suspicion of this rare malignancy.