Nonvalvular atrial fibrillation is the most common sustained arrhythmia in adults, and it is described as a relationship between it and right atrium thrombosis. A case of a 76–year–old man who presented with severe recent-onset dyspnea and several co–morbidities, such as permanent atrial fibrillation, with no anticoagulant therapy is reported here. Echocardiography showed a massive thrombus in right atrium, without any clinical or echographic signs of peripheral veins thrombosis. This article is also a review of the cases from literature.