A 3-year prospective study of road traffic accidents was carried out in Oxfordshire by a combined team of surgeons and engineers. This liaison permitted the comparison of injuries with details of the collisions. Twelve cases of rupture of the thoracic aorta were recorded and in 10 cases the lesion was just distal to the origin of the left subclavian artery. The mean velocity change during the impacts was 38.3 mph, and it is suggested that oblique and transverse forces to the chest as well as rapid deceleration are aetiological factors in this condition.