Cor triatriatum is a rare congenital anomaly characterized by an embriologic defect in which the common pulmonary vein fails to connect to the left posterior atrial wall. As a result, the left atrium is divided by a fibromuscular membrane in 2 chambers, the posterosuperior chamber that receives the drainage of the pulmonary veins, and the anteroinferior chamber that continues with the mitral valve. We report on a 45-year-old woman whose diagnosis was made because of exertional dyspnea and paroxistic atrial fibrillation. Echocardiography showed a characteristic linear echodense structure crossing the left atrium (Figure 1A). Magnetic resonance and right catheterization confirmed the diagnosis of cor triatriatum (Figure 1B,C,D). The patient underwent surgical correction.