Early postoperative patch rupture is a catastrophic complication of carotid endarterectomy reconstruction with greater saphenous vein. Mechanical determinants of saphenous vein rupture were identified by structural measurements and the results applied to carotid endarterectomy patch geometry. Diameter and rupture pressure was measured in fresh saphenous vein segments from the ankle, knee, or thigh in 157 patients undergoing bypass operations. Circumferential hoop rupture stress was calculated and the results were applied to 157 carotid endarterectomy reconstructions. All vein ruptures were in the cylindric axis. The mean vein diameter was 4.58 mm. The mean vein rupture pressure was 2873 mm Hg (3.78 atm). Vein diameter was larger in the thigh than in the ankle or knee (p less than 0.01), but there was no significant difference in rupture pressure between veins from the three locations. Women had a smaller vein diameter than had men at all locations (p less than 0.01). There was a positive linear correlation between vein diameter and rupture pressure. The mean maximum diameter of curvature of 157 carotid endarterectomy reconstructions with a vein patch was 13.3 mm. Multiple random applications of the 157 veins to 157 carotid diameters predicted a mean patch rupture pressure of 1087 mm Hg (1.43 atm), 1163 mm Hg (1.53 atm) for men, and 866 mm Hg (1.14 atm) for women. Predicted vein patch rupture pressures less than 300 mm Hg were found in 5.7% of cases (8.8% women and 1.2% men). Only 0.6% of patients (1.8% women and 0% men) had a predicted rupture pressure less than 200 mm Hg. No veins with a diameter greater than or equal to 4.0 mm had a predicted patch rupture pressure less than 300 mm Hg. These results suggest that small-diameter saphenous veins have a higher risk of rupture when used as a carotid patch.