The first report on the use of modified bovine arteries as vascular substitutes in the dog was published in 1956,1 with the first implantation in man being carried out six years later. Prior to obtaining Food and Drug Administration approval for this biologic "prosthesis," seven-year observations were published from our laboratory,2 and various clinical investigators reported their results in man.3,4 It is now more than 16 years since the onset of these studies and certain conclusions are emerging from our experiences and those of other investigators, published and unpublished. Under conditions approaching those seen in the laboratory, ie, in human iliac artery replacement or bypass, where the vessel is relatively large and the runoff adequate, a very high (almost 100%) long-term success rate is seen. The longest follow-up in man has been six years and nine months. The graft, available after death of the patient from other