A 40-year-old man was admitted to our Department for paroxysmal atrial fibrillation. A transthoracic echocardiographic examination revealed a membrane attached laterally to the junction between the left upper pulmonary vein and left atrial appendage, dividing the left atrium into two chambers: the former communicating with the mitral valve and the latter with pulmonary veins. One fenestration of ∼10 mm connected the two chambers and an echo colour-Doppler demonstrated a flow through this fenestration ( Panel A ; see Supplementary material online, Video S1 ). The mean pressure gradient across the membrane was 5 mmHg, while the maximum gradient was 12 mmHg. The echocardiogram also showed a dilation of the left atrial chamber (volume 59 mL/m2), a slight dilation of the pulmonary veins, a reduced mitral flow, and a mild mitral valve regurgitation due to mitral valve prolapse. …