THROMBOENDARTERECTOMY and synthetic arterial substitution are the two methods of operative therapy currently used for aortoiliac arterial occlusive disease. This study was undertaken to assess the relative effectiveness of these methods of treatment among patients operated on for symptoms of aorto-iliac occlusive disease in the same institution. Material and Methods Two hundred and six patients underwent 216 operations for aorto-iliac arterial occlusive disease in the Barnes Hospital from January 1957 through December 1965. Synthetic arterial substitution was employed in 57 of the operative procedures and thromboendarterectomy in 159. The operations were performed by both the attending and resident staffs. No rigid criteria were used to determine the operative procedure used on a particular patient. The choice was dependent for the most part upon the experience and preference of the surgeon. The endarterectomies were performed at a uniform annual frequency throughout the entire period of study. Synthetic grafts were first used