Deterioration of the aortic wall resulting in formation of aneurysm may be evoked by increased activity of elastases, collagenases and lysosomal proteases. These enzymes come from macrophages and neutrophil granulocytes which are elements of the inflammatory reaction accompanying aneurysm. These cells may also come from parietal thrombus in the aneurysm lumen. The aim of this work was to determine activity of elastase, cathepsin G, collagenase-like Pz-peptidase and cathepsins A, B, C, D and E in the parietal thrombus of aortic aneurysm. The thrombus was obtained from the lumen of the aortic aneurysm of six patients during operation. Protease activities were determined using specific substrates at optimum pH. Retracted blood clot was a comparative material. The thrombus of aortic aneurysm showed two-five fold higher activity of elastases, collagenase-like Pz-peptidase and cathepsins A, D and G in comparison to the blood clot (P < 0.001). However, activity of cathepsins B, C and E in the thrombus was only slightly higher (P < 0.05). Prolonged effect of proteases coming from parietal thrombus on the aneurysm wall could evoke marked degradation of fibrillar proteins resulting in increase of aneurysm.