Objectives: Cathepsin S (Cat S) protein expression is increased in human abdominal aortic aneurysm (AAA) lesions and Cat S has been suggested a direct role by promoting inflammatory response partly in experimental AAA. The purpose of this study is to observe the expression of serum Cat S and hs-CRP and its clinical significance in AAA patients. Design and methods: We collected serum samples from 31 AAA patients and 32 controls. Cat S and hs-CRP levels were measured by a sandwich-type enzyme-linked immunosorbent assay (ELISA) and an enhanced immunoturbidimetric assay respectively. The maximum diameter of the AAA was identified by ultrasonography. Results: The patients with MA had higher serum Cat S and hs-CRP levels than the controls (p < 0.05). Furthermore, human serum Cat S levels were strongly correlated with hs-CRP by the nonparametric Spearman correlation tests (B = 0.849, p < 0.05). Based on Pearson's correlation test, human serum Cat S and hs-CRP levels were positively correlated with AAA diameter size (p <0.05). Cat S was correlated independently with the hs-CRP in all subjects (p < 0.01). After adjustment for the maximum diameter of the abdominal aorta-associated variables, Cat S combined hs-CRP (R-2 = 0.801) is better than Cat S (R-2 = 0.740) in predicting the maximum diameter of AAA lesions. Conclusion: Combined serum Cat S and hs-CRP levels are better in predicting the inflammatory activity of AAA lesions in the clinical setting. (C) 2013 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.