Objective To assess apoptosis mediated by the cell death receptor Fas in peripheral T lymphocytes of patients with abdominal aortic aneurysm. Methods The apoptotic pathway was triggered by anti-Fas monoclonal antibody in cultured and activated peripheral T-cells from 20 AAA patients. Control groups consisted of 15 patients with aortic atherosclerotic occlusive disease(AOD)and 25 healthy individuals. Cell survival and death rate were assessed. Results Cross-linkage of Fas receptor exerted a strong apoptotic response on T cells from AOD patients and healthy controls, while the effect on T cells was very limited from that of AAA patients. The evaluation of cell Survival rate showed a significantly higher percentage in AAA group(98.9%±10.3%)than in the AOD subjects(58.9%±15.2%)or the healthy group(59.4%±12.9%;P＜0.001＝.Apoptosis assessment by annexin V and propidium iodide staining and flow cytometry showed similar results. The defect in AAA group was not due to decreased fas expressed at normal levels. Moreover,it specifically involved the Fas system because cell death was induced in the normal way by methylprednisolone. Conclusions Fas-induced apoptosis in activated T cell from AAA patients is impaired. This may disturb the normal down-regulation of the immune response and thus provide a new insight into possible mechanisms and routes in the pathogenesis of AAA.