Matrix metalloproteinases (MMPs) degrade extracellular matrix and may play a central role in the pathogenesis of aortic aneurysms. We studied 2 groups of patients: 15 with dilatative pathology of the ascending thoracic aorta and 17 with aneurysm of the abdominal aortic wall (AAA). We compared the expression of MMPs, tissue inhibitors of matrix metalloproteinases (TIMPs), and osteopontin in the wall of thoracic and abdominal aneurysms. In AAA, MMP-9 and TIMP-1 expression in inflammatory cells was higher than in smooth muscle cells (SMCs) (median score: 3.5 versus 1, P < .0001; 2 versus 1, P < .04, respectively), whereas MMP-2 demonstrated higher expression in SMCs than in inflammatory cells (median score: 0 versus 4, P < .0001). In ATA, MMP-2, MMP-9, TIMP-1, TIMP-2, TIMP-3, and osteopontin expression in SMCs was higher than in inflammatory cells (median score: 3 versus 0, P < .0001; 4 versus 1, P < .0005; 2 versus 0, P < .001; 5 versus 2, P < .0001; 2 versus 0, P < .005; and 5 versus 1.5, P < .0001, respectively), when both inflammatory cells of the media and the adventitia were considered together. The cellular expression of MMP-9 and their tissue inhibitors TIMP-1, TIMP-2, and TIMP-3 differs in the dilatative pathology of abdominal and thoracic aortas, so the hypothetical model of morphogenesis of AAA cannot completely explain the formation of dilatative pathology of the ascending thoracic aorta.