Cor triatriatum sinistrum is a rare congenital cardiac abnormality often accompanied by other malformations. Transthoracic echocardiography does not always define the membrane and associated abnormalities; so, transoesophageal colour Doppler echocardiography was used to visualize intra-atrial blood flow through the intra-atrial membrane in a patient in whom the membranous orifice could not be seen by precordial echocardiography. There was no anomalous pulmonary venous drainage or associated atrial septal defect.