The study aimed to assess the level of plasma Endothelin-1 (ET-1) in patients before and after transcatheter closure of atrial septal defect (ASD) and to evaluate the usefulness of measuring ET-1 levels for the diagnosis and selection of candidates for ASD closure. 80 patients (55 F, 25 M), mean age 42,2 ± 11,5 years were enrolled for an attempt at ASD closure. A group of 19 healthy volunteers, (12 F, 7 M) mean age 39.2 ± 9.15 served as controls. All ASD patients underwent: clinical and echocardiographic study and cardiopulmonary exercise test. ET-1 levels were measured before and after closure. Whole blood was collected from femoral artery and vein and from pulmonary artery during cardiac catheterization. ET-1 levels at peripheral artery and vein in ASD patients were significantly higher than in the volunteers (p < 0.0001). The ASD subjects with highest ET-1 level presented the larger area of right ventricle and right atrium and higher pulmonary artery systolic pressure(p < 0.05). The ASD subjects with lower ET-1 level demonstrated longer time of exercise and higher peak oxygen consumption (p < 0.05). There was a decrease of ET-1 at peripheral artery (5.128 ± 8.8 vs. 2.22 ± 6.2; p < 0.001) and at peripheral vein (4.401 ± 3.33 vs. 2.05 ± 1.35; p < 0.001) within 48 hours after ASD closure, as compared to the baseline data. After 6 and 12 months farther drop in ET-1 level was observed. 1. The level of ET-1 in ASD patients is elevated in compare to healthy subject.2. The significant reduction of ET-1 level is observed after percutaneous closure of ASD.3. Elevated level of ET-1 in patients with ASD is associated with right heart enlargement.4. Measurements of ET-1 may be a supplemental diagnostic tool and may be helpful in establishing indications for defect closure.