The incidence of bacterial colonization and the microflora of prosthetic vascular graft material explanted from 44 patients undergoing graft revision was determined. Graft material for culture was obtained from aortofemoral or femoropopliteal vascular prostheses without signs of infection but requiring revision for femoral anastomotic aneurysm (n = 21) or thrombosis (n = 26). Explanted graft material was placed in tryptic soy broth and ultrasonically oscillated to disrupt the adherent graft surface biofilm, which is a technique that increases the recovery of microorganisms compared with standard microbiologic culture methods. Microorganisms were isolated from 90% (19/21) of grafts associated with anastomotic aneurysms and 69% (18/26) of thrombosed grafts. Staphylococcus epidermidis was the prevalent organism recovered and accounted for 69% of the isolates. Slime production, a growth characteristic of S. epidermidis associated with prosthetic device infection in humans, was demonstrated by 87% (13/15) of strains isolated from grafts with pseudoaneurysms compared with 33% (4/12) of strains isolated from occluded grafts (p less than 0.01). Despite the high incidence (79%) of colonization of vascular prostheses, no patient developed wound or graft infection after graft replacement and perioperative antibiotic administration. Low-virulent microorganisms, such as S. epidermidis, can colonize vascular prostheses and not provoke signs of graft infection. The high recovery rate of slime-producing S. epidermidis strains from grafts with anastomotic aneurysms and the adherence-mediated growth of these bacteria on biomaterials suggest this late graft complication is the sequelae of a bacteria-laden biofilm infection.