Introduction: Unilateral lung agenesis is a rare congenital anomaly, with an estimated incidence in 1 in 10000~15000 births. It is frequently accompanied by multiple abnormalities in the cardiovascular, facial, skeletal, genitourinary, gastrointestinal and CNS systems. Subject with unilateral lung agenesis can be threatened his life when the airway lumen is compressed externally, which warrants immediate intervention. Case: a 2-month- old boy with right lung agenesis visited ER due to dyspnea and cyanosis. He showed irritability and poor oral intake. He had signs of dyspnea, suprasternal and subcostal retraction, perioral cyanosis and tachypnea with a respiratory rate of 60. His initial Saturation of O2 was 80%. He had mild stridor but did not present crackles or wheezings. His respiratory distress was not improved despite oxygen supply via nasal prong and epinephrine inhalation. High flow oxygen supply could not save him from the respiratory distress and hypercapnia. He ended up with being transferred to ICU. Bronchoscopy revealed his tracheomalacia. Chest CT showed focal but severe tracheal narrowing where the trachea was stuck between the aortic arch and vertebral body. After the aortopexy, an anchoring aortic arch to the anterior chest wall, his symptoms improved dramatically. CT scan showed a relieved compression at his proximal trachea. Since his oxygen saturation was managed above 95% in the room air, he was successfully discharged. Conclusion: A 2 month-old boy was in respiratory distress due to tracheal compression superimposed to the unilateral lung agenesis. Despite supportive cares, his respiratory distress has not improved until we conducted an aortopexy. External airway compression should be explored and corrected as one of the common accompanying medical problems in subjects with unilateral lung agenesis.