A case of an 11-months-old girl with vomiting and laryngeal stridor is presented, and in whom a Kommerell diverticulum was demonstrated, which is a rare variant of the incomplete vascular ring. It is well known that the magnetic resonance is the best study to define this malformation. In this patient, it was the cardiac catheterization which allowed to define the structures that constituted the vascular ring. The patient was surgically treated in a successful way. The embryological knowledge of the transformations of the aortic arches offers great assistance in the understanding of all the types of vascular rings.