Anomalous left pulmonary artery (vascular sling) is a congenital anomaly in which the vascular structure arises either from the posterior surface of the right pulmonary artery, or from the main pulmonary artery and courses to the left lung between the posterior surface of the trachea and the anterior surface of the esophagus. It may cause compression on the tracheobronchial tree. It is a rare condition leading to death in the first months of life, if it is not corrected. Its diagnosis is quite difficult because it usually presents non specific symptoms frequently associated to diffuse tracheal stenosis. The authors present a clinical case of a newborn with trisomy 21 who had a left pulmonary artery sling associated to tracheal stenosis and congenital heart disease (complete atrioventricular septal defect). They review the literature, particular in what concerns embryopathogenesis, the difficulties in establishing the diagnosis and the surgery which must be performed as soon as possible.