Cor triatriatum dexter is a rare congenital malformation in which a membrane divides the right atrium into two chambers. The membrane represents a persistence of the right sinus venosus valve (RSV). Normally the RSV regresses between the 9th and 15th week of gestation, as the cephalic portion forms the crista terminalis and the caudal portion develops into the Eustachian and Thebesian valve. Any failure in the regression process may result in remnants of RSV as a simple muscle bar, a Chiari-network or a fenestrated or unfenestrated membrane (cor triatriatum dexter). We describe a patient with cor triatriatum dexter in whom diagnosis was made several years after successful valvulotomy procedure for severe congenital valvular pulmonary stenosis.