Classic cor triatriatum sinistrum is a rare cardiac malformation to be found in the adult. One year after surgical correction of an atrial septal defect in a 55-year-old man, cor triatriatum sinistrum was diagnosed by transesophageal echocardiography. We compared transthoracic echocardiography, transesophageal echocardiography, and cardiac catheterization to dynamic three-dimensional echocardiography (3-D echo) which offers a new, noninvasive approach to determine the opening size between the accessory and the true left atrium. The findings by 3-D echo, confirmed by left heart catheterization, showed that the accessory membrane was not stenosing. Surgical correction was therefore not indicated in this patient.