Five cases of long-term complications of a homogeneous series of 78 aorto-iliac-femoral by-passes in knitted Dacron velour observed between 1976 and 1988 are reported. The cases were aneurysm of a prosthetic branch and pseudoaneurysms of aortic and femoral anastomoses. The paper concerns not so much the presentation of the causes, recognised by most Authors, that might have determined the ongoing pathology as the observation of a histologic situation not previously described: namely the presence of macrophages which attack Dacron fibres and fragment them.