Deterioration of the aortic wall resulting in formation of aneurysms may be caused by increased activity of metalloproteases and lysosomal proteases. The aim of this work was the evaluation of cathepsin D and cathepsin L activities, and activities of inhibitors of cysteine cathepsins in the wall of aortic aneurysms and in parietal thrombus. Aortic aneurysms were obtained during operation. Aortas taken from organ donors and blood clots were used as control material. Activities of cathepsin D and cathepsin L in the aortic aneurysm wall and parietal thrombus were higher than in the control groups. The aneurysm wall showed lower activity of inhibitors of cysteine proteases than the normal aorta. Parietal thrombus had a higher level of cysteine protease inhibitor activity than blood clot. Cathepsin D and cathepsin L present in the aneurysm wall and in the parietal thrombus filling the aneurysm may act on proteins determining elasticity and mechanical resistance of arteries.