Late occlusion of the saphenous vein graft appears to result in part from wall thickening as an adaptation to increased mean wall stress. Using an established pig model of arteriovenous bypass grafting, the effect of reducing wall stress with an external porous polytetrafluoroethylene stent was investigated. Segments of autologous saphenous vein were implanted by end-to-end anastomoses into both carotid arteries, with one graft supported by a stent 4 mm in diameter. Increases in graft wall dimensions were quantified 4 weeks later by computer-aided planimetry of transverse histological sections. The contribution of hyperplasia (i.e., cell proliferation) to the changes observed was further clarified by measurements of DNA concentration. All grafts showed an increase in external size, but this was restricted by stenting. All grafts also showed an increase in cross-sectional area of the tunica media and tunica intima that was only partly accounted for by an increase in DNA concentration, which indicated that both hyperplasia and hypertrophy had occurred. Stented grafts showed less enlargement of the media but greater enlargement of the intima. Overall wall size was therefore similar in stented and unstented grafts. Stented grafts showed less increase in DNA concentration than unstented grafts. In stented grafts, the residual luminal cross-sectional area was significantly less than in unstented grafts. The data show that external stenting reduces medial enlargement and hyperplasia but increases encroachment of the intima into the lumen. Because final luminal size is thought to be of paramount importance in maintaining long-term patency, external stenting is unlikely to be of benefit.