Blade atrial septostomy (BAS) for pulmonary hypertension has increased long-term survival and is an effective and palliative preliminary to heart and/or lung transplantation. We treated an 18-year-old woman with severe pulmonary primary hypertension whose symptoms had worsened as a resulted low cardiac output. The patient's right ventricular pressure was 150/23 mmHg, cardiac index (CI) 1.0 L/min per m2, and she showed signs and symptoms of severe primary pulmonary hypertension. We performed BAS successfully, paying particular attention to the following points. To maintain pulmonary blood flow after creating an atrial right-to-left shunt, the patient was infused intravenously with packed red blood cells and volume expander. Oxygen delivery was also increased by the transfusion of packed red blood cells. To avoid unacceptable hypoxemia immediately after the procedure, the atrial septum was initially incised with a very small-blade catheter. Nine months after the BAS, catheterization revealed a decrease in mean pulmonary arterial pressure to 73 mmHg and an increase in CI to 2.5 L/min per m2. Thirteen months after the BAS, the patient died as a result of progressive worsening of right-sided heart failure. We concluded that BAS could be successful in patients with severe pulmonary hypertension providing attention is paid to the patient's condition and that BAS is an effective therapy for prolonging survival.