We present the case of a patient with atrial septal defect and severe pulmonary hypertension with pulmonary artery peak pressure greater than 110 mm Hg. Open lung biopsy was done prior to the corrective operation, and pathological findings in the small pulmonary arteries included "musculoelastosis" and complete occlusion of 70% of these small arteries and arterioles. The atrial septal defect was closed, and long-term oral prostacyclin therapy was initiated. Pulmonary artery peak pressure decreased to 65 mm Hg 2 years after the operation. This case demonstrates that in a patient with 70% complete occlusion of small pulmonary arteries and arterioles resulting from "musculoelastosis," not only is surgical intervention possible but also pulmonary artery pressure decreases in the long term after operation.