Objective: Many Down syndrome patients have atrial septal defect (ASD) and associated pulmonary hypertension. Sometimes ASD closure of Down syndrome patients who have severe pulmonary hypertension is controversial due to the concern of exacerbation of pulmonary hypertension. The aim of this study is to know about clinical ourcome after ASD closure in Down syndrome children Methods: We retrospectively reviewed Down syndrome patients’ medical records who have done ASD patch closures at the Seoul National University Children’s Hospital since January 2000. Results: Total 15 patients were operated for ASD. Nine patients were diagnosed as pulmonary hypertension before ASD patch closure by echocardiography. Three patients took multiple medication (Bosentan, sildenafil) for severe pulmonary hypertension. The mean age of ASD patch closure is 17.3 month ( 2month to 11yrs) and the mean size of ASD is 10.2 mm (4.9mm to 15.8mm). Three patients who took medications for severe pulmonary hypertension underwent ASD patch closure at the age of 7 month,12 month and 25 month. Each patient continued medication for 13 month, 6 month, 21 month further after ASD closure and pulmonary hypertension did not recur after discontinuance of pulmonary hypertension medications. Other 12 patients also did not show pulmonary hypertension after ASD closure. Conclusion: Even Down syndrome patients who had severe pulmonary hypertension with large ASD could be operated without exacerbation of pulmonary hypertension with multiple medications of pulmonary hypertension.