We report a case of cor triatriatum in a 4-year-old girl on whom precise anatomic diagnosis was made preoperatively by cardiac catheterization and angiocardiography. The anomalous septum was removed through successful surgery. The condition had been diagnosed two years earlier as pulmonary hypertension secondary to a ventricular septal defect, in spite of a high pulmonary artery wedge pressure (PAWP). We emphasize the importance of the PAWP in separating pulmonary venous hypertension from other forms of pulmonary hypertension since this subgroup is usually operable and even curable if surgery is performed before irreversible pulmonary vascular changes occur. In spite of the small risk, we feel that angiocardiographic studies are necessary to demonstrate the anomalous septum.