PURPOSE: To determine the value of the CT scan in distinguishing an ischemic and a tumoral segment in coloniccarcinoma complicated by proximal bowel ischemia. MATERIALS AND METHODS: CT scans of twenty patients with ischemic colitis proximal to obstructing colonic carcinoma were reviewed retrospectively. The presence of anischemic segment proximal to colonic carcinoma were patho-logically confirmed in 12 patients, and the remaining eight patients showed typical radiologic findings of bowel ischemia on barium enema but on pathologic review showed only colonic carcinoma. CT scans were analyzed for the location, wall thickness, length, and enhancing pattern of both tumoral and ischemic segments in correlation with barium enema or surgico-pathologic results. Theresults of tumor staging shown on CT scan were compared with those of pathologic findings. RESULTS: On CT scan adistinction between ischemic and tumoral segments could be made in 15 patients (75%). The ischemic segments were contiguously proximal to the tumoral segment in 18 patients. In two patients, however, there was an intervening segment of normal bowel between the two segments and this was confirmed by pathology. Maximvm bowel wall thickness ranged from 0.8 to 4.5cm (mean, 2.0cm) in tumoral segments and from 0.6 to 1.5 cm (mean, 1.0cm) in ischemic segments (p<0.05). Tumoral segments were enhanced heterogeneously in 12 patients (60%) and homogeneously in the remaining eight, while ischemic segments were enhanced homogeneously in 14 patients (70%) and heterogeneously insix. Peripheral rim enhancement was seen only in the ischemic segments of four patients (20%). Comparing TNM tumorstaging of the CT scan with that of pathology, CT scan overstaged in two patients (10%) and understaged in one(5%). CONCLUSION: CT is a valuable tool for distinguishing an ischemic from a tumoral segment in patients with ischemic colitis proximal to colonic carcinoma. An understanding of this pathologic entity could reduce the possibility of over or understaging in cases of colonic carcinoma.