PURPOSE: To evaluate CT findings for the differential diagnosis of mechanical bowel obstruction and paralytic ileus. MATERIALS AND METHODS: Without information relating to clinical or operative findings, we retrospectively analyzed the CT scans of 24 patients with mechanical bowel obstruction and 19 patients with paralytic ileus. Final diagnosis was confirmed by operation (n=26), or by clinical symptoms, radiologic findings and follow-up study CT findings were obtained : 1) the diameter of the most dilated part of the small bowel, and the thickness and enhancing pattern of the dilated small bowel wall; 2) the diameter of the most dilated part of the descending colon and the ratio of the diameter of the small bowel to that of the descending colon; 3) the number of transitional zones, length and thickness. and 4) associated ascites and its location. RESULTS: The mean diameters of the most dilated part of the small bowel in mechanical bowel obstruction and paralytic ileus were 3.6cm and 2.9cm, respectively. The diameter of the small bowel in mechanical bowel obstruction was significantly greater than in paralytic ileus(p< .05). The mean thickness of dilated small bowel wall was 4.0mm in mechanical bowel obstruction and 2.4mm in paralytic ielus, and target-like enhancement was prominent in mechanical bowel obstruction (46%) (p< .05). he mean diameter of the most dilated part of the descending colon was not significantly different to that of the most dilated part of the small bowel, but the ratio of the diameter of the small bowel to that of the colon was 2.9 in mechanical bowel obstruction and 1.9 in paralytic ileus, respectively, which was statistically significant (p< .05). A transitional zone was seen in 23 cases (96%) of mechanical bowel obstruction and in nine (47%) of paralytic ileus. In mechanical bowel obstruction, mean transitional zone length was 2cm, shorter than that of paralytic ileus (3.4cm) (p< .05) The thickness of transitional zone and the presence of ascites and its locations were not significantly different between mechanical bowel obstrction and paralytic ileus. CONCLUSION: In the differential diagnosis of mechanical bowel obstruction and paralytic ileus, the following CT findings were considered useful : diameter of the most dilated part of the small bowel ; thickness and target-like enhancing pattern of dilated small bowel wall ; ratio of the diameter of the small bowel to that of the descending colon ; and the number of transitional zones, and their length.