Symptomatic pulmonary arterial hypertension (PAH) in patients with isolated atrial septal defect (ASD) is rare during infancy. We report a case of isolated ASD with severe PAH in an infant who developed airway obstruction as cardiomegaly progressed. The patient presented with recurrent severe respiratory insufficiency and failure to thrive before the repair of the ASD. Echocardiography confirmed volume overload on the right side of heart and severe PAH (tricuspid regurgitation [TR] with a peak pressure gradient of 55 to 60 mmHg). The chest radiographs demonstrated severe collapse of both lung fields, and a computed tomography scan showed narrowing of the main bronchus because of an intrinsic cause, as well as a dilated pulmonary artery compressing the main bronchus on the left and the intermediate bronchus on the right. ASD patch closure was performed when the infant was 8 months old. After the repair of the ASD, echocardiography showed improvement of PAH (TR with a peak pressure gradient of 22 to 26 mmHg), and the patient has not developed recurrent respiratory infections while showing successful catch-up growth. In infants with symptomatic isolated ASD, especially in those with respiratory insufficiency associated with severe PAH, extrinsic airway compression should be considered. Correcting any congenital heart diseases in these patients may improve their symptoms.