Indication and treatment of adult secundum ASD with severe PH was discussed in two surgical cases. Case 1: 32-year-old woman with dyspnea on exercise. After 100% O2 inhalation, PA systolic pressure changed from 103 to 104 mmHg, left to right shunt ratio changed from 18.2% to 52.2%, Qp/Qs changed from 1.07 to 2.08, Rp/Rs changed from 0.58 to 0.28, PVR changed from 1606 to 805 dyne.sec.cm-5 respectively. Because the reversibility of pulmonary obstructive disease was recognized, ASD closure was performed. Her activity became well, and PVR was decreased to 758 after the operation. Case 2: 63-year-old man with dyspnea on exercise. After 100% O2 inhalation, PA systolic pressure changed from 102 to 95 mmHg, Left to right shunt ratio changed 31.9 to 51.0%, Qp/Qs changed from 1.30 to 2.0, Rp/Rs changed from 0.65 to 0.39, PVR changed from 1257 to 806 dyne.sec.cm-5. After the operation, he got well, and PA systolic pressure was decreased to 85 mmHg, PVR was 620. 100% O2 inhalation test was valuable to evaluate the reversibility of pulmonary obstructive state and to decide the surgical indication.