With few exceptions, the incidence of left colon ischemia following abdominal aortic reconstruction was reported to be 1-2%. All reports of such ischemic events are retrospective analyses of clinically manifest or autopsy cases. Fifty patients were studied prospectively to determine more accurately the incidence of this complication. Aortic reconstruction was performed in 23 patients for occlusive disease (OD) and in 27 for aneurysmal disease (AD). No emergency operations were performed. All patients underwent colonoscopy within 4 days of operation. Three instances of colon ischemia were noted, an incidence of 6% (OD 4.3%, AD 7.4%). Each patient recovered uneventfully. Two patients had diarrhea, but only after colon ischemia was recognized. Arteriographic opacification of the inferior mesenteric artery by the superior mesenteric artery collateral (meandering mesenteric artery) was documented in 35% of patients with OD and in 27% of patients with AD. Colon ischemia did not develop when this collateral was identified. The inferior mesenteric artery was patent as the aorta in all who developed colon ischemia. Although clinically significant colitis following aortic reconstruction is rare, colonoscopy after operation may prove to be valuable for early recognition of ischemic changes before clinical manifestations preclude effective management.