Intrinsic prosthetic failure is quite uncommon since the advent of Dacron prostheses. Only 24 cases of arterial prosthetic failure have been described in the literature. Our experience consists of two additional failures of knitted Dacron prostheses. The first patient developed aortojejunal fistula due to a defect of the aortic prosthesis 6 years after resection and replacement of an abdominal aortic aneurysm. The analysis of the graft revealed a defect in the form of a dropped stitch, which caused a weak spot where the prosthesis was able to be torn apart longitudinally. The second patient had femoropopliteal bypass and 6 years later developed degeneration with aneurysmal formation of the entire length of the prosthesis. Optical microscopy of this prosthesis exhibited mostly transverse failures which were coincident with valleys of the corrugated prosthesis. This was indicative of a defect that rendered the corrugation root too weak to withstand normal arterial pressures. This presentation emphasizes the seldom and serious complication of prosthetic failure due to defect of the prosthetic material. Details of scanning electron microscopy of the prosthesis, clinical presentation, angiography, and pathological findings with treatment also are discussed.