A 40-year-old man presented with a 2-month-long history of fever and chest pain. Transthoracic echocardiogram (A in systole, B in diastole) revealed that the dissection flap arose from the posterior wall of left atrium and separated the left atrium into 2 cavities: the true cavity, and false cavity (*). An abnormal left ventricular-atrial tunnel (arrow) can be found in the posterior part of mitral valve annulus. Transesophageal echocardiogram demonstrated that no communication of blood flow presented across the interlayer flap and only mild mitral insufficiency occurred (C in systole, D in diastole, Online Videos 1 and 2). Multi-slice computed tomography images showed the dissected left atrial wall (E), and pathological findings confirmed that the flap was the intima of left atrium (F). The mechanism of this condition is unclear. To our knowledge, this is the only case confirmed by medical images and pathological findings.