Since aortic aneurysms (AAs) are mostly asymptomatic, but they have a high mortality rate upon rupture, their detection and progression evaluation are clinically important issues. To discover diagnostic biomarkers for AA, we performed proteome analysis of aortic media from patients with thoracic atherosclerotic AA (TAAA), comparing protein levels between the aneurysm and normal tissue areas. After hierarchical clustering analysis of the proteome analysis data, tissue samples were classified into three groups, regardless of morphological features. This classification was shown to reflect disease progression stage identified by pathological examination. This proteomics-based staging system enabled us to identify more significantly altered proteins than the morphological classification system. In subsequent data analysis, Niemann-Pick disease type C2 protein (NPC2) and insulin-like growth factor-binding protein 7 (IGFBP7) were selected as novel biomarker candidates for AA and were compared with the previously reported biomarker, thrombospondin 1 (THBS1). Blood concentrations of NPC2 and IGFBP7 were significantly increased, while THBS1 levels were decreased in TAAA and abdominal atherosclerotic AA patients. Receiver operating characteristic analysis of AA patients and healthy controls showed that NPC2 and IGFBP7 have higher specificity and sensitivity than THBS1. Thus, NPC2 and IGFBP7 are promising biomarkers for the detection and progression evaluation of AA. Aortic aneurysm (AA) is a disease of aortic dilation that results in a bulge or swelling in the aorta. AAs are mostly asymptomatic, but they have a high mortality rate upon dissociation or rupture 1-3. The early detection and progression evaluation of AA are clinically urgent issues, because surgical treatments, such as the implantation of stent grafts or aortic prosthesis, are available 4,5. AA is often detected by chance in diagnostic imaging, for example, by computed tomography, magnetic resonance imaging, or ultrasonic echo for other diseases, or during voluntary health examination 6. However, highly sensitive and specific diagnostic tests for the detection of AA have not yet been developed. D-dimer and C-reactive protein (CRP) levels in the blood have been reported to be suitable for the diagnosis of AA 7-9. However, these markers have poor disease specificity and effectiveness due to their primary design and aims, and do not reflect aortic tissue degeneration during the formation and progression of the aneurysm 10,11. Therefore, novel biomarkers based on the molecular mechanism of the development and progression of AA are highly desired for the diagnosis of AA. A major cause of aortic aneurysm is atherosclerosis. In the advanced atherosclerosis lesion, aortic wall structure is usually disrupted and embrittled. Fragile aortas are then enlarged, resulting in an aneurysm 12. Although the pathogenic mechanism of atherosclerosis is not fully understood, it is well recognized that endothelial cell dysfunction occurs as a first step, followed by the accumulation...