Thirty-nine synthetic (32 Dacron and 7 Teflon) arterial prostheses (from 38 patients with peripheral arterial disease) removed after periods between 2 months and 18 years, were examined by histology and immuno-histology. The grafts were initially permeated by thrombus containing platelet antigens and this became organised and converted to granulation, and then to fibrous, tissue. The newly-formed tissue contained 'foreign-body' giant-cells in contact with the plastic prosthesis and showed evidence of permeation by plasma proteins. In grafts of over 2 years duration, this reactive tissue no longer contained platelet antigens but invariably revealed bound lipid, identifiable as apolipoprotein-B-containing lipoproteins (LpB), and fibrinogen-related antigens (FRA), in a distribution resembling that seen in atherosclerotic arteries. LpB and FRA were also seen in organised, or partially organised, mural thrombi in older grafts. The oldest grafts additionally showed stenosis, calcification or aneurysm formation. Lipid deposition increases with the age of grafts; is independent of the nature of the plastic fibre used or its mode of fabrication; and sometimes contributes to graft failure. Immuno-histology indicates that this is an insudative process indistinguishable from 'true' atherosclerosis which occurs in graft-linings of prostheses of long duration and in old mural thrombi in grafts and that the lipid in these lesions derives from plasma LpB rather than from platelets. This source for the lipid suggests that the insudative and thrombogenic theories of atherogenesis can be reconciled.