A 26-year-old man was evaluated for progressive shortness of breath on mild exertion. Physical examination was unremarkable. The electrocardiogram revealed left atrial (LA) enlargement. Transthoracic echocardiography (TTE) and cardiovascular magnetic resonance (CMR) demonstrated normal biventricular size and systolic function with severely dilated LA and no significant valvulopathy. A thin mobile complex septated membrane was noted within the LA with no clear fenestrations (Fig. 1A and B, Supplementary movie 1 and 2). Transesophageal echocardiogram (TEE) revealed that the membrane is attached to the lateral wall at the coumadin ridge with color turbulence at the site of attachment yet no fenestrations were seen. Medially, the membrane was attached to the inter-atrial septum just above the fossa ovalis with possible multiple fenestrations by color Doppler. The calculated peak/mean gradients were 13-14/9-10 mmHg respectively at both ends. A septum within the membrane was visualized extending from its lateral wall and attached to the inter-atrial septum just above the fossa ovalis (Fig. 1C, D and 2, Supplementary movie 3-7). A decision was made to proceed with surgical correction. The findings of the TTE/TEE/CMR were confirmed and the atrial membrane was excised around its periphery. Recovery from the surgery was uneventful and he was asymptomatic 6 months afterwards. Echocardiography revealed neither residual membrane nor stenosis of the pulmonary veins.