Nineteen cases of colon ischemia following surgery for abdominal aortic aneurysm are reported. Thirteen patients developed bloody diarrhea within 72 hours after surgery: nine died from acute gangrene of the colon; two survived with hemicolectomies; and two survived with intact colons. The etiology is most likely arterial insufficiency based on postoperative anatomy or traumatic venous thrombosis. The use of antibiotics preoperatively did not prevent acute colon gangrene. Prompt recognition is imperative and surgery is the treatment of choice. Six patients developed diarrhea from six to eight days postoperatively. Proctoscopic findings varied from normal to those resembling ulcerative colitis. Barium enemata revealed mucosal spasm and edema near the splenic flexure in five patients, which later disappeared. Cultures showed no pathogenic bacteria. Rectal biopsies all showed fibrin thrombi in arterioles. All patients survived and were free of bowel complaints.