Computed tomography (CT) was used to study 42 patients with rectal carcinoma. Water was used as a contrast medium for studying the local extent of tumor in all patients. Scans were read prospectively without knowledge of the histologic staging and then compared with pathologic specimens. CT depicted the tumor in all patients. Comparison of CT and histologic results (following the Dukes classification) showed that disease was correctly staged as A in three of four patients, as B in eight of 12, as C in 15 of 17, and as D in nine of nine. Overall, carcinoma was correctly staged with CT in 35 of 42 patients (diagnostic accuracy, 83.3%). The accuracy in the assessment of local invasion was 97.6% (41 of 42). In the detection of lymph node involvement, the accuracy was 78.6% (sensitivity, 88%; specificity, 64.7%). CT is recommended in the preoperative staging of rectal carcinoma and as an aid in choosing the appropriate therapy. The use of water enema and complete distention of the rectum are reliable techniques for improving the accuracy of CT in the assessment of local invasion by cancer.