Ageing has a striking impact on heart and vascular system, and particularly on the large elastic arteries, i.e. aorta, determining a multitude of changes at different structural and functional levels. As result, medial degeneration (MD) occurs. A characteristic example of MD is sporadic thoracic aortic aneurysm (S-TAA), whose patho-physiological mechanisms remain unclear. In this study, typical MD morphological phenotypes were researched in S-TAA case and control aorta specimens by histopathological and immunohistochemical analyses. Three phenotypes (I, II and III) were detected, but the phenotype III was mainly observed. Elevated cystic MD, plurifocal medial apoptosis and increased metalloproteinase-9 amount characterize it. In addition, it was significantly correlated with the severity of elastic fragmentation, hypertension, smoking, and particularly with advancing age. Thus, the phenotype III might represent the typical MD phenotype associated in S-TAA in old people having major risk of aorta rupture and dissection independently on aneurysm diameter. This might permit to assume the phenotype III with its typical histological abnormalities as optimal biomarker of rupture and/or dissection in aged individuals useful both to apply different surgical approaches and to provide appropriate surgical indications.