Summary Dogs with unpurified (brown) Teflon thoracic aortic grafts were followed for a period of five years. During this period the grafts were completely functional. On gross examination, no degenerative changes were noted. Microscopically, however, moderate areas of focal calcification and rarely osseous metaplasia were observed. An active process of collagenization invests the grafts with a vascularized adventitia and a relatively acellular type of intima. There appears to have been no significant increase in degenerative changes in these dogs observed over five years, when compared with dogs observed for shorter periods. This finding suggests that, if additional degenerative changes occur beyond five years, they must indeed proceed at an extremely slow rate. These findings tend to substantiate earlier observations regarding the attributes of Teflon as a vascular prosthetic.