Cor triatriatum sinister is a rare congenital cardiac anomaly in which the pulmonary venous confluence is separated from \"true\" left atrium by fibro-muscular septum. Cor triatriatum can persist unrecognized into adult life if large opening or several fenestrations are present. Late onset of symptoms is usually caused by increased pulmonary pressure, development of atrial arrhythmias or mitral valve abnormalities. Non obstructive cor triatriatum may be an incidental finding. We describe a case of an adult woman who had known cor triatriatum (but not being actively followed) who presented in palpitation acces table due to atrial fibrillation.