A case of suppurative aortitis, probably due to Proteus vulgaris, associated with idiopathic cystic medial necrosis is reported. Through the inflamed area, the aorta had perforated into the left pleural cavity and the left lung and had dissected through the media into the arch. The association of suppurative aortitis and diseases of the aortic wall such as congenital anomalies, syphilitic mesaortitis, atheroma and idiopathic cystic medial necrosis is briefly discussed. It is concluded that the occurrence of perforation and dissection depends on the nature of the underlying lesion.