RESUMEN
Cor triatrium sinister is a rare congenital disease [0,1% of cases of congenital heart disease]. Cor triatrium is recognized by the finding of an abnormal fibromuscular membrane that subdivise the left atrium into posterosuperior and anteroinferor chambers. This anomaly creates an obstacle to the venous pulmonary flow. Depending on the severity of obstruction, cor triatrium may be symptomatic in childhood but it can be symtom free even in adulthood. Diagnosis of this abnormality has been easy supported by transthoracic and transoesophageal echocardiography. Treatment is easy to perform and consists in surgical definitive correction in symptomatic patients. We report a case of a 31-year-old man in whom a non obstructive cor triatrium was discovered while evaluation for an atrial fibrillation secondary to a hyperthyroidism. According to this rare case we try to evaluate the severity and the outcome of this disease relating the literature data