An aortic arch extending into the neck is a rare congenital anaomaly. Left-sided cervical aortic arch was thought to be much less common than right-sided, but they are now almost equal in incidence in the published reports. This paper describes a case of left-sided cervical aortic arch in an adult, with a previously undescribed association, aneurysm of the descending thoracic aorta. Cervical aortic arch must be considered in the differential diagnosis of pulsatile cervical swellings. The embryological development of the aortic arch is discussed. This is still controversial and it is not possible to be sure of the embryological explanation of this congenital anomaly of the aortic arch from a study of the adult anatomy.