It is unusual that an atrial flutter reveals a tumor of the atrium. Two cases are reported: one occurred in a patient with a myxoma of the left atrium, the other was found in a patient with a non-hodgkinian lymphoma of the right atrium. The diagnosis was made possible by sonocardiography in both cases. The transesophageal view improves the quality of the images. MRI provided further informations regarding the lymphoma. Modern techniques of cardiac imaging permit the early diagnosis and treatment of cardiac tumors which must be investigated in case of apparently isolated atrial arrhythmias.