Cor triatriatum is equally well imaged by transesophageal echo and magnetic resonance tomography in 1 adult patient. Diagnosis can also be made by transthoracic echocardiography and usually is. The initial diagnosis of classic cor triatriatum is rarely made as late as in the 3rd decade of life. Because the stenotic opening in the membrane separating the accessory from the true left atrium was found to be between 9 and 11 mm in diameter, the onset of symptoms in this patient was very late.