It was studied the role of cyclophillin A in the development of inflammatory reaction in the wall of the abdominal aorta that was the main reason of its aneurysm formation. 197 planned operations were done on abdominal aortic aneurysm. During the operations the fragment of 3×2 cm from the front wall of the aneurysm was excised for the purpose of morphological study. The wall of abdominal aorta under atherosclerosis was used as a control samples for study. It was made the histological, morphometric study of the samples and immunohistochemical revealing of cyclophillin A. The abdominal aorta aneurysms had such characteristic features as chronic inflammation localized in the media and adventitia, the destruction of the elastic framework of the aortic wall and increase of cyclophillin A content in it. The absence of cyclophillin A in the middle and outer layers of the aortic wall in case of atherosclerosis indicates that cyclophillin A is the cause of inflammatory reaction in case of an aneurysm formation. The aforesaid information gives the principled opportunity to control the aneurysm development and to treat the small size aneurysms.