Apparatus and methods for treating a wall of an aneurysm formed in a vessel includes introducing a tubular member into the body lumen until a distal end of the tubular member is located within the aneurysm. A fluid is delivered via a lumen of the tubular member into the aneurysm to at least partially de-endothelialize the wall of the aneurysm, thereby causing an endothelium of the wall to generate fibrous tissue to strengthen the wall of the aneurysm.