A deviation in an indirect measurement of intramural pH below the limits of normality (6.86) was used as a diagnostic test for sigmoid ischemia in 25 high-risk patients undergoing abdominal aortic operations. The clinical diagnosis of ischemic colitis was made by the attending physicians in only two of the 25, on the day after operation in one and three months after operation in another. In neither was the ischemic colitis considered to have been a causative factor in their subsequent deaths. In contrast, six patients developed pH evidence of ischemia on the day of operation. All six subsequently developed a transient episode of guaiac-positive diarrhea, four developed physical signs consistent with ischemic colitis, and four died. Of 19 who did not develop pH evidence of ischemia, none developed guaiac-positive diarrhea, none developed any signs of ischemic colitis, and none died. Stepwise logistic regression showed the duration of pH evidence of ischemia on the day of operation to be the best predictor for the symptoms and signs of ischemic colitis and for death after operation.