Abdominal aortic aneurysm (AAA) is a chronic inflammatory disease with no available pharmacological treatment. AAA formation reduces the structural integrity of the vessel and increases the susceptibility to rupture. The inflammatory response within human aneurysmal tissue is characterized by increased expression of cyclooxygenase-2 (COX-2). Similarly, in a mouse model of the disease induced by chronic Angiotensin II (AngII) infusion, we have shown that COX-2 expression in the abdominal aortic smooth muscle layer increases early in the development of the disease. Furthermore, genetic or pharmacological inactivation of COX-2 prior to disease initiation reduces AAA incidence. The current study utilized nonhyperlipidemic mice to determine the effectiveness of COX-2 inhibition initiated after AAA formation. COX-2 inhibitor treatment was initiated 5 days after beginning the AngII infusion, a time-point where significant aneurysmal pathology is observed. COX-2 inhibition with celecoxib significantly reduced the incidence as well as severity of AAAs as compared to the control group. Celecoxib treatment also protected the mice from aortic rupture and death. AAA development is characterized by degradation of the aortic smooth muscle layer with loss of the contractile phenotype. We found that the effectiveness of celecoxib was associated with significantly increased mRNA expression of alpha-actin, SM22alpha and desmin, all of which are markers of a differentiated smooth muscle cell phenotype. Celecoxib treatment also decreased mRNA expression of a marker of dedifferentiated smooth muscle (hyaluronic acid synthase 2). We also examined the role of altered expression of COX-2 in the increased susceptibility of the abdominal segment to AAA formation. We found a prolonged and greater induction of COX-2 in the abdominal aortic smooth muscle layer in contrast to a transient induction of COX-2 in the other regions of the aorta throughout disease progression. Overall, these findings suggest that COX-2 plays an important role in AAA development in mice, and COX-2 inhibition with celecoxib attenuates progression of aneurysm development by maintaining a differentiated phenotype in abdominal aortic smooth muscle cells.