Background: Natural killer (NK) cells have an emerging role in the development of chronic disease and in the direction and maintenance of inflammatory responses. Abdominal aortic aneurysms (AAA) is a chronic inflammatory disorder of unknown aetiology. The aim was to investigate whether NK cells showed altered function in patients with an AAA. Methods: The presence, phenotype and function of peripheral blood and tissue NK cells from patients with an AAA, peripheral vascular disease (PVD) and healthy age-sex-matched controls were assessed before and after surgery. Results: Patients with an AAA had significantly higher (P <0.010) percentages of peripheral blood NK cells (mean (95 per cent c.i.) 23.8 (2.6) per cent) than patients with PVD (17.4 (2.9) per cent) and control subjects (16.2 (2.8) per cent). The NK cells from patients with an AAA had increased cytotoxicity on a per cell basis towards both an NK-sensitive target cell line and human aortic smooth muscle cells. Increased NK cell proportions (22.7 (3.5) per cent) and cytotoxic activity, together with higher C-reactive protein values, persisted after successful AAA repair. Conclusion: These data support the hypothesis that increased NK cytotoxicity could be a contributing factor in the generation or potentiation of inflammation in patients with an AAA. Copyright © 2005 British Journal of Surgery Society Ltd.