Background/objectives Pulmonary arterial hypertension is an important complication in hemodynamically relevant atrial septal defects (ASD) and negatively affects outcome. This retrospective study aimed at (1) estimating the prevalence of pulmonary hypertension (PH) in patients with secundum ASD and (2) identifying predictors of PH development or persistence after ASD closure.Methods Consecutive patients with an isolated secundum ASD from the Belgian Registry on Adult Congenital Heart Disease were studied. Demographic, clinical, echocardiographic and invasive hemodynamic measurements were analyzed. PH was defined upon the echocardiographic PH probability (tricuspid regurgitation velocity ≥ 2.9 m/s).Results PH prevalence in the entire ASD population (295 patients, 68.8% females, mean age 46 ± 21 years) was 15.9% compared to 13.3% in patients after ASD closure. PH after ASD closure was significantly related to mortality (p = 0.001), atrial arrhythmia (p 55 years), PH prevalence was the highest (34%) and mean pulmonary artery pressure (mPAP) at catheterization before was related to PH after closure (HR 1.09). Twenty patients in the PH group had mPAP