We report an uncommon case of large ostium secundum atrial septal defect (ASD) with severe pulmonary arterial hypertension causing aneurysmal dilatation of the pulmonary arteries (PAs) leading to compression of the left main bronchus and collapse of the entire left lung in a 15-month-old female child. The patient was managed by surgical closure of the ASD, translocation of the right PA anterior to the aorta with PA aneurysmorrhaphy. Left bronchial compression was relieved with complete lung expansion on the third postoperative day.