Cor triatriatum dexter is a rare congenital disorder caused by persistence of the right valve of the embryonic sinus venosus that results in septation of the right atrium. Its hemodynamic significance is determined by the extent of the subdivision of the right atrium and the number and size of connections between the 2 right atrial subchambers, and on associated cardiac anomalies that may be present. In patients with cor triatriatim dexter, there usually is elevation of the right atrial pressure. It can cause functional stenosis that impedes forward flow with potentially lethal consequences. We report a case of cor triatriatum dexter in a patient with hemodynamic instability in whom a Swan-Ganz catheter could not be floated. It was not detected by transthoracic echocardiography; however, transesophageal echocardiography clarified the diagnosis, which led to hemodynamically successful percutaneous intervention.