Background: Atrial septostomy is a palliative treatment for patients with pulmonary hypertension (PHTN) refractory to vasodilator therapy. Limited data exist in the pediatric population and in patients with repaired congenital heart defects. Methods: We performed a retrospective analysis of hemodynamic and symptomatic changes in patients with PHTN who underwent an atrial septostomy at our institution. Results: Forty-six atrial septostomies were performed on 43 patients. Patient ages ranged from 0.3 to 30 years (median 12.5). Of 43 patients, 29 had primary PHTN, 10 had PHTN associated with repaired congenital heart defects, and 4 had other secondary causes of PHTN. Mean baseline pulmonary vascular resistance was 35 ± 17 Wood units, and mean pulmonary artery pressure was 74 ± 19 mm Hg. Patients surviving ≥30 days had immediate improvement in cardiac index (from 2.3 to 2.9 L · min-1 · m-2, P < .0001), right atrial pressure (RAp) (from 9.9 to 8.3 mm Hg, P < .05), and oxygen delivery (from 424 to 491 mL O2 per minute, P < .01), with a decrease in systemic oxygen saturation (from 93% to 86%, P < .001). Pulmonary artery pressure was unchanged (P = .3). New York Heart Association class and symptoms of syncope improved (P < .01). Event-free survival at 1, 2, and 3 years was 84%, 77%, and 69%, respectively. Using the National Institutes of Health Registry model, predicted survival probability significantly improved (P < .001). Ten patients (22%) died within 30 days of catheterization. Mortality was associated with preceding decompensations in the intensive care unit (6/10, P < .001) and a higher RAp (21.4 vs 9.8 mm Hg, P < .001). Conclusions: Atrial septostomy provides symptomatic and hemodynamic improvement in cardiac index and RAp. Risk of septostomy is increased in patients with preceding decompensation or a RAp >18 mm Hg. © 2007.