BACKGROUND & AIMS: Approximately one third of patients who present to gastroenterology care with constipation have rectal evacuation disorders. We aimed to compare rectal gas volume, measured by computerized tomography (CT), in constipated patients with and without rectal evacuation disorders. METHODS: In a retrospective study, wecollected data from1553 patients with constipation, evaluated over 20 years. We analyzed data from 141 patients evaluated by anorectal manometry, balloon expulsion tests, and colon transit tests, collecting records of abdominal and pelvic CT examinations. Patients were classified into 3 subgroups: those with rectal evacuation disorders, slow-transit constipation, or normal-transit constipation. Two observers used standard CT software to identify variable regions of interest on each cross-sectional CT image that contained rectum and measured areas of gas in each slice; they then summated entire volumes of rectal gas. For the 3 groups, we compared rectal gas volume, maximal rectal gas transaxial area (measured by CT), and area of rectal gas (vertical) on the 2-dimensional abdominal film (scout) using the Kruskal-Wallis test. RESULTS: The intraclass correlation coefficient between 2 observers' measurements of rectal gas volume was 0.99 (P < .001). There were overall group differences in rectal gas volume and the maximal rectal gas transaxial area (both P < .001). Themedian rectal gas volumewas higher in patients with rectal evacuation disorders (13.84 cm(3)) than in patients with slow-transit (2.51 cm(3)) or normal-transit constipation (1.33 cm(3), both P < .05). Similarly, the area of rectal gas, which correlated with the maximal rectal gas transaxial area (Spearman correlation coefficient, 0.7; P < .001), showed overall 3-group differences (P =. 033), with greater areas of rectal gas on the abdominal scout film in patients with rectal evacuation disorders than in those with normal-transit constipation. CONCLUSIONS: In an analysis of patients with constipation, we found rectal gas volume, determined by abdominal CT imaging, to be greater in patients with than without rectal evacuation disorders.