BACKGROUND Pulmonary arterial hypertension (PAH) may develop in patients with atrial septal defects (ASD); however, little is known about associated risk factors and its evolution after transcatheter ASD closure. RESULTS We conducted a cohort study on 215 adults with attempted transcatheter ASD closure from 1999 to 2006. Patients were classified according to baseline systolic pulmonary artery pressures as having no (I, <40 mm Hg), mild (II, 40 to 49 mm Hg), moderate (III, 50 to 59 mm Hg), or severe (IV, >or=60 mm Hg) PAH. Independent predictors of moderate or severe PAH were older age (odds ratio [OR], 1.10 per year; P<0.0001), larger ASD (OR, 1.13 per millimeter; P=0.0052), female sex (OR, 3.9; P=0.0313), and at least moderate tricuspid regurgitation (OR, 3.6; P=0.0043). At 15 (interquartile range, 8 to 43) months post-ASD closure, patients with higher baseline pressures were more likely to experience a >or=5-mm Hg decrease (33.7%, 73.9%, 79.2%, and 100.0% in groups I to IV, P<0.0001), with a larger magnitude of reduction (0, 8, 17, and 22 mm Hg; P<0.0001). However, normalization of pressures (<40 mm Hg) occurred less frequently in patients with more advanced PAH (90.2%, 71.7%, 66.7%, and 23.5%, P<0.0001). Among patients with moderate or severe PAH, independent predictors of normalization were lower baseline pressures (OR, 0.91 per mm Hg; P=0.0418) and no more than mild tricuspid regurgitation (OR, 0.14; P=0.0420). CONCLUSIONS In adults with ASDs, severity of PAH is modulated by age, sex, defect size, and degree of tricuspid regurgitation. Patients with moderate or severe PAH may benefit from substantial reductions in pulmonary artery pressures after transcatheter ASD closure, although the PAH values remain elevated in a sizeable proportion.