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A 3 month old boy was referred because of pronounced dyspnoea. Bronchoscopy revealed a normal upper airway and tracheal bifurcation. The left main bronchus was compressed to 80% of its diameter and the lung lobulation was normal. Echocardiography showed a pulmonary artery sling. Frontal angiography of the main pulmonary artery (right) confirmed the anomalous and low origin of a good sized left pulmonary artery (LPA) from the right pulmonary artery (RPA). The patient underwent operative repair under cardiopulmonary bypass with detachment of the aberrant LPA and its implantation into the main pulmonary artery. He is currently doing well without respiratory medication nine months after surgery. In the pulmonary artery sling, supracarinal or carinal crossing of the LPA causes airway obstruction at the level of the trachea. Isolated subcarinal crossing of the LPA causing left main bronchus obstruction like our case is exceptional. Definite diagnosis with delineation of vascular anatomy is made by pulmonary angiography combined or completed with tracheobronchoscopy to demonstrate the airway compression and the associated tracheal anomalies. Echocardiography allows diagnosis of pulmonary artery sling but is not always predictive, and the barium swallow test demonstrates a pulsatile anterior indentation of the oesophagus but does not show the anatomy. Computed tomography and magnetic resonance imaging delineates pulmonary artery sling, but they are often difficult to perform in uncooperative infants.