BACKGROUND Patients with severe primary pulmonary hypertension have a poor prognosis, but those with a patent foramen ovale may survive longer. A few reports of clinical improvement after blade balloon atrial septostomy in patients with severe pulmonary vascular disease have appeared. The purpose of this study was to systematically evaluate the effects of blade balloon atrial septostomy on clinical signs and symptoms, hemodynamics, and survival in patients with severe primary pulmonary hypertension. RESULTS Blade balloon atrial septostomy was performed on 15 children and young adults with severe primary pulmonary hypertension. Despite maximal medical therapy, prior to septostomy all patients had recurrent syncope and 8 had severe right heart failure. Thirteen patients survived the procedure. After blade balloon atrial septostomy, no patient experienced further syncope, and signs and symptoms of right heart failure improved in all New York Heart Association Class IV patients. Within 24 hours after the procedure and at follow-up catheterization 7 to 27 months after septostomy, there was a significant increase in cardiac index, resulting in an increase in systemic oxygen transport. There was improved long-term survival in the 13 patients who survived blade balloon atrial septostomy compared with similar groups of primary pulmonary hypertension patients who received standard therapy (P < .05). CONCLUSIONS Blade balloon atrial septostomy resulted in clinical and hemodynamic improvement and improved survival in selected patients with severe primary pulmonary hypertension.