A 67-year-old woman with a history of rheumatic heart disease presented with dyspnea. She had previously refused the recommended surgery and underwent medical treatment. A chest x-ray revealed massive cardiomegaly with a cardiothoracic ratio of 0.94 (A). Transthoracic echocardiography showed a giant left atrium (GLA) associated with rheumatic mitral valves with calcified tips. The left atrium (LA) measured 18.8 × 14 cm (B, Online Videos 1 and 2), and severe mitral regurgitation was observed in an apical 4-chamber view. Transesophageal echocardiography delineated diastolic doming in the anterior mitral leaflet (AMV), whereas the posterior leaflet (PMV) was almost fixed (C, Online Video 3). Computed tomography revealed massive LA enlargement (19.3 × 14.7 × 18.4 cm) (D, E), and the LA volume was 3,110 ml. A volume-rendering computed tomography scan demonstrated that the GLA was nearly encompassing the whole mediastinum (E, G). This GLA, the largest ever reported to our knowledge, demonstrates the extent to which an LA can expand.