A 38-year-old woman with a known history of rheumatic heart disease for 10 years presented with complaints of breathlessness, dysphagia, and hoarseness of voice for 6 months. On physical examination, she had an apical pansystolic murmur. An electrocardiogram showed atrial fibrillation. Chest radiography in the posteroanterior view revealed a massive cardiomegaly with a cardiothoracic ratio of 0.96 (A). Transthoracic echocardiography showed a hugely dilated left atrium (LA) with severe mitral regurgitation and mild mitral stenosis (B and C, Online Video 1). A computed tomographic scan of the chest showed a giant left atrium measuring 19.5  17.5 mm with compression of the right atrium (RA), right ventricle (RV), left ventricle (LV), and adjacent structures (D and E). Three-dimensional volume-rendered imaging showed a massive dilation of the left atrium occupying the entire mediastinum (F, Online Video 2). She underwent successful mitral valve replacement with left atrial reduction and Maze procedure. A giant left atrium has been almost exclusively described in rheumatic heart disease and is due to pancarditis with eccentric dilation, which can cause compression of adjacent structures. AO ¼ aorta; PA ¼ pulmonary artery